Graphene foam-based protective layer for an anode-less alkali metal battery

ABSTRACT

Provided is a lithium or sodium metal battery, comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between the cathode and the anode, wherein the anode comprises: (a) an anode current collector, initially having no lithium, lithium alloy, sodium or sodium alloy as an anode active material supported by the anode current collector when the battery is made and prior to a charge or discharge operation; and (b) a graphene foam, comprising multiple pores and pore walls, wherein the graphene foam either substantially constitutes the anode current collector or is disposed between the anode current collector and the electrolyte and wherein the graphene foam, when tested under compression, has a recoverable elastic deformation or compressibility from 5% to 150%.

FIELD

The present disclosure relates generally to the field of alkali metal battery (e.g. lithium metal battery or sodium metal battery) and, more particularly, to a lithium or sodium metal secondary battery having a graphene foam-based protective layer and a process for producing this protective layer and the battery.

BACKGROUND

Lithium-ion and lithium (Li) metal cells (including Lithium-sulfur cell, Li-air cell, etc.) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium metal has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound (except Li_(4.4)Si) as an anode active material. Hence, in general, rechargeable Li metal batteries have a significantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having high specific capacities, such as TiS₂, MoS₂, MnO₂, CoO₂ and V₂O₅, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were dissolved from the lithium metal anode and transferred to the cathode through the electrolyte and, thus, the cathode became lithiated. Unfortunately, upon cycling, the lithium metal resulted in the formation of dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.

Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. These issues are primarily due to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway. Many attempts have been made to address the dendrite-related issues, as briefly summarized below:

Fauteux, et al. [D. Fauteux, et al., “Secondary Electrolytic Cell and Electrolytic Process,” U.S. Pat. No. 5,434,021, Jul. 18, 1995] applied to a metal anode a protective surface layer (e.g., a mixture of polynuclear aromatic and polyethylene oxide) that enables transfer of metal ions from the metal anode to the electrolyte and back. The surface layer is also electronically conductive so that the ions will be uniformly attracted back onto the metal anode during electrodeposition (i.e. during battery recharge). Alamgir, et al. [M. Alamgir, et al. “Solid polymer electrolyte batteries containing metallocenes,” U.S. Pat. No. 5,536,599, Jul. 16, 1996] used ferrocenes to prevent chemical overcharge and dendrite formation in a solid polymer electrolyte-based rechargeable battery.

Skotheim [T. A. Skotheim, “Stabilized Anode for Lithium-Polymer Battery,” U.S. Pat. No. 5,648,187 (Jul. 15, 1997); U.S. Pat. No. 5,961,672 (Oct. 5, 1999)] provided a Li metal anode that was stabilized against the dendrite formation by the use of a vacuum-evaporated thin film of a Li ion-conducting polymer interposed between the Li metal anode and the electrolyte. Skotheim, et al. [T. A. Skotheim, et al. “Lithium Anodes for Electrochemical Cells,” U.S. Pat. No. 6,733,924 (May 11, 2004); U.S. Pat. No. 6,797,428 (Sep. 28, 2004); U.S. Pat. No. 6,936,381 (Aug. 30, 2005); and U.S. Pat. No. 7,247,408 (Jul. 24, 2007)] further proposed a multilayer anode structure consisting of a Li metal-based first layer, a second layer of a temporary protective metal (e.g., Cu, Mg, and Al), and a third layer that is composed of at least one layer (typically 2 or more layers) of a single ion-conducting glass, such as lithium silicate and lithium phosphate, or polymer. It is clear that such an anode structure, consisting of at least 3 or 4 layers, is too complex and too costly to make and use.

Protective coatings for Li anodes, such as glassy surface layers of LiI—Li₃PO₄—P₂S₅, may be obtained from plasma assisted deposition [S. J. Visco, et al., “Protective Coatings for Negative Electrodes,” U.S. Pat. No. 6,025,094 (Feb. 15, 2000)]. Complex, multi-layer protective coatings were also proposed by Visco, et al. [S. J. Visco, et al., “Protected Active Metal Electrode and Battery Cell Structures with Non-aqueous Interlayer Architecture,” U.S. Pat. No. 7,282,295 (Oct. 16, 2007); U.S. Pat. No. 7,282,296 (Oct. 16, 2007); and U.S. Pat. No. 7,282,302 (Oct. 16, 2007)].

Despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. Solid electrolytes typically have a low lithium ion conductivity, are difficult to produce and difficult to implement into a battery.

Furthermore, solid electrolyte, as the sole electrolyte in a cell or as an anode-protecting layer (interposed between the lithium film and the liquid electrolyte) does not have and cannot maintain a good contact with the lithium metal. A big physical gap tends to be created between the Cu foil surface (or lithium metal layer surface) and the solid electrolyte. This reduces the effectiveness of the electrolyte to support dissolution of lithium ions (during battery discharge), transport lithium ions, and allowing the lithium ions to re-deposit back to the lithium anode (during battery recharge).

Another major issue associated with the lithium metal anode is the continuing reactions between electrolyte and lithium metal, leading to repeated formation of “dead lithium-containing species” that cannot be re-deposited back to the anode and become isolated from the anode. These reactions continue to irreversibly consume electrolyte and lithium metal, resulting in rapid capacity decay. In order to compensate for this continuing loss of lithium metal, an excessive amount of lithium metal (3-5 times higher amount than what would be required) is typically implemented at the anode when the battery is made. This adds not only costs but also a significant weight and volume to a battery, reducing the energy density of the battery cell. This important issue has been largely ignored and there has been no plausible solution to this problem in battery industry.

Clearly, an urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries, and to reducing or eliminating the detrimental reactions between lithium metal and the electrolyte.

Among various advanced energy storage devices, alkali metal batteries, including Li-air (or Li—O₂), Na-air (or Na—O₂), Li—S, and Na—S batteries, are especially attractive due to their high specific energies. However, sodium metal batteries suffer from similar problems. The Li—O₂ battery is possibly the highest energy density electrochemical cell that can be configured today.

The Li—O₂ cell has a theoretic energy density of 5.2 kWh/kg when oxygen mass is accounted for. A well configured Li—O₂ battery can achieve an energy density of 3,000 Wh/kg, 15-20 times greater than those of Li-ion batteries. However, current Li—O₂ batteries still suffer from poor energy efficiency, poor cycle efficiency, and dendrite formation and penetration issues.

One of the most promising energy storage devices is the lithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode. The lithium-sulfur cell operates with a redox couple, described by the reaction S₈+16Li↔8Li₂S that lies near 2.2 V with respect to Li⁺/Li^(∘). This electrochemical potential is approximately ⅔ of that exhibited by conventional positive electrodes (e.g. LiMnO₄). However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Assuming complete reaction to Li₂S, energy densities values can approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on the combined Li and S weights or volumes. If based on the total cell weight or volume, the energy densities can reach approximately 1,000 Wh/kg and 1,100 Wh/l, respectively. However, the current Li-sulfur cells reported by industry leaders in sulfur cathode technology have a maximum cell specific energy of 250-350 Wh/kg (based on the total cell weight), which is far below what is possible. In summary, despite its great potential, the practical realization of the Li—S battery has been hindered by several obstacles, such as dendrite-induced internal shorting, low active material utilization efficiency, high internal resistance, self-discharge, and rapid capacity fading on cycling. The most serious problem of Li metal secondary (rechargeable) batteries remains to be the dendrite formation and penetration. Sodium metal batteries have similar dendrite problems.

SUMMARY

A specific object of the present disclosure is to provide graphene foam-based protective layers for lithium metal and sodium metal secondary batteries that exhibit long and stable charge-discharge cycle life without exhibiting lithium or sodium dendrite problems.

The present disclosure provides a graphene foam-based protective layer for an alkali metal battery (lithium or sodium metal battery) and a process for producing such a protective layer and the battery. The disclosure also provides a lithium or sodium metal battery containing such a protective layer in the anode.

In a preferred embodiment, the disclosure provides a lithium or sodium metal battery, the battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between the cathode and the anode, wherein the anode comprises: (a) an anode current collector, initially having no lithium, lithium alloy, sodium or sodium alloy as an anode active material supported by the anode current collector when the battery is made and prior to a charge or discharge operation; and (b) a reversibly compressible graphene foam, comprising multiple pores and pore walls, wherein the graphene foam either substantially constitutes the anode current collector (the graphene foam layer itself is part or entirety of a current collector) or is disposed between the anode current collector and the electrolyte and wherein the graphene foam, when tested under compression, has a recoverable elastic deformation (or reversible compressibility) greater than 5% (more typically from 10% to 95%, and most typically from 20% to 80%). This graphene foam structure may be compressed (deformed under a compressive force) to a great extent, but can snap back to substantially the original dimensions when the force is released. This is a reversibly compressible foam structure.

Preferably, the graphene foam layer is subjected to a compression of at least 5% (more preferably >10% and most preferably >20%) when the battery cell is manufactured. When the graphene foam is made, it is in a naturally relaxed state. It is advantageous to compress the graphene foam layer during or after the battery cell is made. This would enable the graphene foam layer to maintain a good physical contact with the anode current collector, the lithium metal layer (when present), or a solid state electrolyte.

In some embodiments, the pore walls contain single-layer or few-layer graphene sheets, wherein the few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 2.0 nm (more typically <1.0 nm and most typically <0.7 nm) as measured by X-ray diffraction. The single-layer or few-layer graphene sheets can contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.001% to 25% by weight of non-carbon elements wherein the non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene (e.g. nitrogen-doped graphene), chemically functionalized graphene, or a combination thereof.

In some embodiments, the graphene foam is chemically bonded to a surface of the anode current collector, such as a Cu foil. The anode current collector may be selected from, for instance, a Cu foil, a Cu-coated polymer film, a sheet of Ni foam, a porous layer of nano-filaments, such as graphene sheets, carbon nanofibers, carbon nano-tubes, etc.

The graphene foam is preferably chemically bonded or physically attached to a surface of the anode current collector and maintains an electric contact with the current collector.

A porous separator may not be necessary if the electrolyte is a solid-state electrolyte.

In certain desired embodiments, this reversibly compressible graphene foam may be a thin layer (having a thickness from 10 nm to 50 μm, more typically from 30 nm to 30 μm) disposed against a surface of an anode current collector. In such a battery cell, the anode contains a current collector without a lithium metal or sodium metal or any other anode active material, such as graphite or Si particles, when the battery cell is manufactured. Such a battery cell having an initially lithium metal-free and sodium-free anode is commonly referred to as an “anode-less” lithium battery or sodium battery. For illustration purpose and using an anode-less lithium battery as an example, the lithium ions that are required for shuttling back and forth between the anode and the cathode are initially stored in the cathode active materials (e.g. Li in LiMn₂O₄ and LiMPO₄, where n=Ni, Co, F, Mn, etc.). During the first battery charge procedure, lithium ions (Li⁺) come out of the cathode active material, move through the electrolyte and then get deposited on a surface of the graphene foam and/or a surface of the anode current collector. As this charging procedure continues, more lithium ions get deposited onto the current collector surface or into pores of the graphene foam, eventually forming a lithium metal film or coating.

During the subsequent battery discharge, this lithium film or coating layer decreases in thickness due to dissolution of lithium into the electrolyte to become lithium ions, creating a gap between the current collector and a solid-state electrolyte or between the current collector and a protective layer if the protective layer were not elastic. Such a gap would make the re-deposition of lithium ions back to the anode impossible during a subsequent recharge procedure. We have observed that the compressible graphene foam is capable of expanding or shrinking congruently or conformably with the anode layer. This capability or reversible compressibility helps to maintain a good contact between the current collector (or the lithium film itself) and the protective layer, enabling the re-deposition of lithium ions without interruption. The same advantages were observed for sodium batteries similarly configured.

In certain embodiments, the present disclosure provides an alkali metal battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between the cathode and the anode, wherein the anode comprises: (a) an anode current collector, an anode active material supported by the anode current collector, wherein the anode active material is selected from lithium, lithium alloy, sodium, sodium alloy, or a combination thereof; and (b) a reversibly compressible graphene foam, comprising multiple pores and pore walls, wherein the graphene foam either substantially constitutes the anode current collector or is in electronic contact with the anode current collector and wherein the graphene foam, when tested under compression, has a recoverable elastic deformation or reversible compressibility greater than 5%; wherein the anode active material resides in pores of the graphene foam (e.g. deposited on pore wall surfaces) or in physical contact with the graphene foam.

In certain embodiments, the alkali metal battery further comprises a polymer that is disposed between the graphene foam and the electrolyte (or electrolyte-separator assembly), impregnated into the pores, or coated on or bonded to surfaces of the pore walls, wherein the polymer comprises an elastomer, an elastic polymer, an electron-conducting polymer, an ion-conducting polymer, or a combination thereof. This polymer preferably forms a discrete layer, a thin film, or a thin coating (preferably having a thickness from 5 nm to 10 μm) that protects lithium metal from direct contact with the electrolyte. We have observed that such a contact could induce undesirable side reactions that consume active lithium and electrolyte.

The elastomer preferably comprises a polymer selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene,-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, a copolymer thereof, a sulfonated version thereof, or a combination thereof

The elastic polymer refers to a polymer that is capable of reversibly deforming from 5% to 800% when tested under tension. Upon release of the tension or tensile force, the polymer recoils or snap back to substantially its original dimensions. The elastic polymer preferably contains a cross-linked network of polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.

The ion-conducting polymer is preferably selected from poly(ethylene oxide) (PEO), polypropylene oxide (PPO), poly(ethylene glycol) (PEG), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), cyanoethyl poly(vinyl alcohol) (PVACN), polyacrylic acid (PAA), a pentaerythritol tetraacrylate (PETEA)-based polymer, an aliphatic polycarbonate (including poly(vinylene carbonate) (PVC), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and poly(trimethylene carbonate) (PTMC)), a single Li-ion conducting solid polymer electrolyte with a carboxylate anion, a sulfonylimide anion, or sulfonate anion, a crosslinked electrolyte of poly(ethylene glycol) diacrylate (PEGDA) or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

The electron-conducting polymer preferably comprises chains of a conjugated polymer selected from polyacetylene, polythiophene, poly(3-alkylthiophenes), polypyrrole, polyaniline, poly(isothianaphthene), poly(3,4-ethylenedioxythiophene), alkoxy-substituted poly(p-phenylene vinylene), poly(2,5-bis(cholestanoxy) phenylene vinylene), poly(p-phenylene vinylene), poly(2,5-dialkoxy) paraphenylene vinylene, poly[(1,4-phenylene-1,2-diphenylvinylene)], poly(3′,7′-dimethyloctyloxy phenylene vinylene), polyparaphenylene, polyparaphenylene, polyparaphenylene sulfide, polyheptadiyne, poly(3-hexylthiophene), poly(3-octylthiophene), poly(3-cyclohexylthiophene), poly(3-methyl-4-cyclohexylthiophene), poly(2,5-dialkoxy-1,4-phenyleneethynylene), poly(2-decyloxy-1,4-phenylene), poly(9,9-dioctylfluorene), polyquinoline, a derivative thereof, a copolymer thereof, a sulfonated version thereof, or a combination thereof.

In certain embodiments, the elastomer, elastic polymer, electron-conducting polymer or ion-conducting polymer further contains a lithium salt dispersed therein (in the polymer) and the lithium salt is preferably selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

In one preferred embodiment, the graphene foam is pre-loaded with lithium or sodium before the battery is made, or the anode further contains a lithium source or sodium source. The lithium source is preferably selected from foil, particles, or filaments of lithium metal or lithium alloy having no less than 80% by weight of lithium element in the lithium alloy. The sodium source is preferably selected from foil, particles, or filaments of sodium metal or sodium alloy having no less than 80% by weight of sodium element in the sodium alloy.

In some embodiments, each lithium or sodium metal battery cell contains an anode layer wherein the graphene foam structure itself is an anode current collector that supports lithium when the battery is charged. Thus, the anode electrode is a one-layer structure containing no additional, separate current collector (such as the commonly used Cu foil). In some alternative embodiments, the lithium metal battery further comprises a separate, discrete anode current collector in contact with the compressible graphene foam. Typically, there is a separate, discrete cathode current collector (e.g. Al foil) in contact with the cathode active material layer (containing cathode active material, such as MoS₂, TiO₂, V₂O₅, LiV₃O₈, S, Se, etc.), which is supported by (coated on) the Al foil.

The few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 0.40 nm as measured by X-ray diffraction and the single-layer or few-layer graphene sheets contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having an adjustable 0.001% to 25% by weight of non-carbon elements wherein said non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof.

The graphene foam typically has a density from 0.005 to 1.7 g/cm³ (preferably from 0.1 to 1.7 g/cm³), a specific surface area from 50 to 2,500 m²/g (more typically from 300 to 1,500 m²/g), a thermal conductivity of at least 200 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 2,000 S/cm per unit of specific gravity. Preferably, the average pore size in the hybrid foam is from 2 nm to 50 nm, and the specific surface area is from 500 m²/g to 1,500 m²/g.

In the graphene foam layer, the pore walls typically contain a 3D network of interconnected graphene planes. In some embodiments, the foam has a physical density higher than 0.8 g/cm³ and a specific surface area greater than 800 m²/g. In certain embodiments, the graphene-carbon hybrid foam has a physical density higher than 1.0 g/cm³ and a specific surface area greater than 500 m²/g. In certain embodiments, the graphene foam is chemically or physically activated to further increase the specific surface area.

The present disclosure also provides a process for producing a graphene foam of an interconnected network of graphene sheets.

It may be noted that there are no limitations on the shape or dimensions of the presently invented graphene foam. In a preferred embodiment, the integral graphene foam is made into a continuous-length roll sheet form (a roll of a continuous foam sheet) having a thickness no less than 100 nm and no greater than 10 cm and a length of at least 1 meter long, preferably at least 2 meters, further preferably at least 10 meters, and most preferably at least 100 meters. This sheet roll is produced by a roll-to-roll process. There has been no prior art graphene-based foam that is made into a sheet roll form. It has not been previously found or suggested possible to have a roll-to-roll process for producing a continuous length of graphene foam, either pristine or non-pristine based.

For battery electrode applications, the graphene foam preferably has an oxygen content or non-carbon content less than 1% by weight, and the pore walls have stacked graphene planes having an inter-graphene spacing less than 0.35 nm, a thermal conductivity of at least 250 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 2,500 S/cm per unit of specific gravity.

In a further preferred embodiment, the graphene foam has an oxygen content or non-carbon content less than 0.01% by weight and said pore walls contain stacked graphene planes having an inter-graphene spacing less than 0.34 nm, a thermal conductivity of at least 300 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 3,000 S/cm per unit of specific gravity.

In yet another preferred embodiment, the graphene foam has an oxygen content or non-carbon content no greater than 0.01% by weight and said pore walls contain stacked graphene planes having an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.7, a thermal conductivity of at least 350 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 3,500 S/cm per unit of specific gravity.

In still another preferred embodiment, the graphene foam has pore walls containing stacked graphene planes having an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.4, a thermal conductivity greater than 400 W/mK per unit of specific gravity, and/or an electrical conductivity greater than 4,000 S/cm per unit of specific gravity.

In a preferred embodiment, the pore walls contain stacked graphene planes having an inter-graphene spacing less than 0.337 nm and a mosaic spread value less than 1.0. In a preferred embodiment, the graphene foam exhibits a degree of graphitization no less than 80% (preferably no less than 90%) and/or a mosaic spread value less than 0.4. In a preferred embodiment, the pore walls contain a 3D network of interconnected graphene planes.

In a preferred embodiment, the solid graphene foam contains meso-scaled pores having a pore size from 2 nm to 50 nm. The solid graphene foam can also be made to contain micron-scaled pores (1-500 μm).

There is no limitation on the methods of producing graphene foams that can be used as a compressible protecting layer. This can be a catalytic CVD (followed by acid etching of Ni foil), hydrothermal reduction of GO hydrogel, and the use of a sacrificial material (e.g. colloidal polystyrene particles, PS) that is coated with graphene oxide sheets using a self-assembly approach (followed by dissolving PS).

Advantageously, the presently disclosed solid graphene foam may be produced by a process comprising:

-   (a) preparing a graphene dispersion having a graphene material     dispersed in a liquid medium, wherein the graphene material is     selected from pristine graphene, graphene oxide, reduced graphene     oxide, graphene fluoride, graphene chloride, graphene bromide,     graphene iodide, hydrogenated graphene, nitrogenated graphene,     chemically functionalized graphene, or a combination thereof and     wherein the dispersion contains an optional blowing agent (foaming     agent); -   (b) dispensing and depositing the graphene dispersion onto a surface     of a supporting substrate (e.g. plastic film, rubber sheet, metal     foil, glass sheet, paper sheet, etc.) to form a wet layer of     graphene material, wherein the dispensing and depositing procedure     includes subjecting the graphene dispersion to an     orientation-inducing stress; -   (c) partially or completely removing the liquid medium from the wet     layer of graphene material to form a dried layer of graphene     material, typically having a content of non-carbon elements (e.g. O,     H, N, B, F, Cl, Br, I, etc.) no less than 5% by weight; and (d) heat     treating the dried layer of graphene material at a first heat     treatment temperature from 100° C. to 3,200° C. at a desired heating     rate sufficient to induce volatile gas molecules from the non-carbon     elements or to activate the blowing agent for producing the solid     graphene foam having a density from 0.01 to 1.7 g/cm³ (more     typically from 0.1 to 1.5 g/cm³, and even more typically from 0.1 to     1.0 g/cm³, and most typically from 0.2 to 0.75 g/cm³), or a specific     surface area from 50 to 3,000 m²/g (more typically from 200 to 2,000     m²/g, and most typically from 500 to 1,500 m²/g).

This optional blowing agent is not required if the graphene material has a content of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.) no less than 5% by weight (preferably no less than 10%, further preferably no less than 20%, even more preferably no less than 30% or 40%, and most preferably up to 50%). The subsequent high temperature treatment serves to remove a majority of these non-carbon elements from the graphene material, generating volatile gas species that produce pores or cells in the solid graphene material structure. In other words, quite surprisingly, these non-carbon elements play the role of a blowing agent. Hence, an externally added blowing agent is optional (not required). However, the use of a blowing agent can provide added flexibility in regulating or adjusting the porosity level and pore sizes for a desired application. The blowing agent is typically required if the non-carbon element content is less than 5%, such as pristine graphene that is essentially all-carbon.

The blowing agent can be a physical blowing agent, a chemical blowing agent, a mixture thereof, a dissolution-and-leaching agent, or a mechanically introduced blowing agent.

The process may further include a step of heat-treating the solid graphene foam at a second heat treatment temperature higher than the first heat treatment temperature for a length of time sufficient for obtaining a graphene foam wherein the pore walls contain stacked graphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 0.40 nm and a content of non-carbon elements less than 5% by weight (typically from 0.001% to 2%). When the resulting non-carbon element content is from 0.1% to 2.0%, the inter-plane spacing d₀₀₂ is typically from 0.337 nm to 0.40 nm.

If the original graphene material in the dispersion contains a non-carbon element content higher than 5% by weight, the graphene material in the solid graphene foam (after the heat treatment) contains structural defects that are induced during the step (d) of heat treating. The liquid medium can be simply water and/or an alcohol, which is environmentally benign.

In a preferred embodiment, the process is a roll-to-roll process wherein steps (b) and (c) include feeding the supporting substrate from a feeder roller to a deposition zone, continuously or intermittently depositing the graphene dispersion onto a surface of the supporting substrate to form the wet layer of graphene material thereon, drying the wet layer of graphene material to form the dried layer of graphene material, and collecting the dried layer of graphene material deposited on the supporting substrate on a collector roller. Such a roll-to-roll or reel-to-reel process is a truly industrial-scale, massive manufacturing process that can be automated.

In one embodiment, the first heat treatment temperature is from 100° C. to 1,500° C. In another embodiment, the second heat treatment temperature includes at least a temperature selected from (A) 300-1,500° C., (B) 1,500-2,100° C., and/or (C) 2,100-3,200° C. In a specific embodiment, the second heat treatment temperature includes a temperature in the range from 300-1,500° C. for at least 1 hour and then a temperature in the range from 1,500-3,200° C. for at least 1 hour.

There are several surprising results of conducting first and/or second heat treatments to the dried graphene layer, and different heat treatment temperature ranges enable us to achieve different purposes, such as (a) removal of non-carbon elements from the graphene material (e.g. thermal reduction of fluorinated graphene to obtain graphene or reduced graphene fluoride, RGF)) which generate volatile gases to produce pores or cells in a graphene material, (b) activation of the chemical or physical blowing agent to produce pores or cells, (c) chemical merging or linking of graphene sheets to significantly increase the lateral dimension of graphene sheets in the foam walls (solid portion of the foam), (d) healing of defects created during fluorination, oxidation, or nitrogenation of graphene planes in a graphite particle, and (e) re-organization and perfection of graphitic domains or graphite crystals. These different purposes or functions are achieved to different extents within different temperature ranges. The non-carbon elements typically include an element selected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron. Quite surprisingly, even under low-temperature foaming conditions, heat-treating induces chemical linking, merging, or chemical bonding between graphene sheets, often in an edge-to-edge manner (some in face-to-face manner).

In one embodiment, the sheet of solid graphene foam has a specific surface area from 200 to 2,000 m²/g. In one embodiment, the sheet of solid graphene foam has a density from 0.1 to 1.5 g/cm³. In an embodiment, step (d) of heat treating the layer of graphene material at a first heat treatment temperature is conducted under a compressive stress. In another embodiment, the process comprises a compression step to reduce a thickness, pore size, or porosity level of the sheet of graphene foam. In some applications, the graphene foam has a thickness no greater than 200 μm.

In an embodiment, the graphene dispersion has at least 3% by weight of graphene oxide dispersed in the liquid medium to form a liquid crystal phase. In another embodiment, the graphene dispersion contains a graphene oxide dispersion prepared by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid in a reaction vessel at a reaction temperature for a length of time sufficient to obtain the graphene dispersion wherein the graphitic material is selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof and wherein the graphene oxide has an oxygen content no less than 5% by weight.

In an embodiment, the first heat treatment temperature contains a temperature in the range from 80° C-300° C. and, as a result, the graphene foam has an oxygen content or non-carbon element content less than 5%, and the pore walls have an inter-graphene spacing less than 0.40 nm, a thermal conductivity of at least 150 W/mK (more typically at least 200 W/mk) per unit of specific gravity, and/or an electrical conductivity no less than 2,000 S/cm per unit of specific gravity.

In a preferred embodiment, the first and/or second heat treatment temperature contains a temperature in the range from 300° C-1,500° C. and, as a result, the graphene foam has an oxygen content or non-carbon content less than 1%, and the pore walls have an inter-graphene spacing less than 0.35 nm, a thermal conductivity of at least 250 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 2,500 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains a temperature in the range from 1,500° C.-2,100° C., the graphene foam has an oxygen content or non-carbon content less than 0.01% and pore walls have an inter-graphene spacing less than 0.34 nm, a thermal conductivity of at least 300 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 3,000 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains a temperature greater than 2,100° C., the graphene foam has an oxygen content or non-carbon content no greater than 0.001% and pore walls have an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.7, a thermal conductivity of at least 350 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 3,500 S/cm per unit of specific gravity.

If the first and/or second heat treatment temperature contains a temperature no less than 2,500° C., the graphene foam has pore walls containing stacked graphene planes having an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.4, and a thermal conductivity greater than 400 W/mK per unit of specific gravity, and/or an electrical conductivity greater than 4,000 S/cm per unit of specific gravity.

In one embodiment, the pore walls contain stacked graphene planes having an inter-graphene spacing less than 0.337 nm and a mosaic spread value less than 1.0. In another embodiment, the solid wall portion of the graphene foam exhibits a degree of graphitization no less than 80% and/or a mosaic spread value less than 0.4. In yet another embodiment, the solid wall portion of the graphene foam exhibits a degree of graphitization no less than 90% and/or a mosaic spread value no greater than 0.4.

Typically, the pore walls contain a 3D network of interconnected graphene planes that are electron-conducting pathways. The cell walls contain graphitic domains or graphite crystals having a lateral dimension (L_(a), length or width) no less than 20 nm, more typically and preferably no less than 40 nm, still more typically and preferably no less than 100 nm, still more typically and preferably no less than 500 nm, often greater than 1μm, and sometimes greater than 10 μm. The graphitic domains typically have a thickness from 1 nm to 200 nm, more typically from 1 nm to 100 nm, further more typically from 1 nm to 40 nm, and most typically from 1 nm to 30 nm.

Preferably, the solid graphene foam contains meso-scaled pores having a pore size from 2 nm to 50 nm (preferably 2 nm to 25 nm). It may be noted that it has not been possible to use Ni-catalyzed CVD to produce graphene foams having a pore size range of 2-50 nm. This is due to the notion that it has not been proven possible to prepare Ni foam templates having such a pore size range and not possible for the hydrocarbon gas (precursor molecules) to readily enter Ni foam pores of these sizes. These Ni foam pores must also be interconnected. Additionally, the sacrificial plastic colloidal particle approaches have resulted in macro-pores that are in the size range of microns to millimeters.

In a preferred embodiment, the present disclosure provides a roll-to-roll process for producing a solid graphene foam composed of multiple pores and pore walls The process comprises: (a) preparing a graphene dispersion having a graphene material dispersed in a liquid medium, wherein the dispersion optionally contains a blowing agent; (b) continuously or intermittently dispensing and depositing the graphene dispersion onto a surface of a supporting substrate to form a wet layer of graphene material, wherein the supporting substrate is a continuous thin film supplied from a feeder roller and collected on a collector roller; (c) partially or completely removing the liquid medium from the wet layer of graphene material to form a dried layer of graphene; and (d) heat treating the dried layer of graphene material at a first heat treatment temperature from 100° C. to 3,000° C. at a desired heating rate sufficient to activate the blowing agent for producing said solid graphene foam having a density from 0.01 to 1.7 g/cm³ or a specific surface area from 50 to 3,000 m²/g.

The orientation-inducing stress may be a shear stress. As an example, the shear stress can be encountered in a situation as simple as a “doctor's blade” that guides the spreading of graphene dispersion over a plastic or glass surface during a manual casting process. As another example, an effective orientation-inducing stress is created in an automated roll-to-roll coating process in which a “knife-on-roll” configuration dispenses the graphene dispersion over a moving solid substrate, such as a plastic film. The relative motion between this moving film and the coating knife acts to effect orientation of graphene sheets along the shear stress direction.

This orientation-inducing stress is a critically important step in the production of the presently invented graphene foams due to the surprising observation that the shear stress enables the graphene sheets to align along a particular direction (e.g. X-direction or length-direction) to produce preferred orientations and facilitate contacts between graphene sheets along foam walls.

Further surprisingly, these preferred orientations and improved graphene-to-graphene contacts facilitate chemical merging or linking between graphene sheets during the subsequent heat treatment of the dried graphene layer. Such preferred orientations and improved contacts are essential to the eventual attainment of exceptionally high thermal conductivity, electrical conductivity, elastic modulus, and mechanical strength of the resulting graphene foam. In general, these great properties could not be obtained without such a shear stress-induced orientation control.

Also provided is a process for producing a continuous sheet of a lithium metal cell electrode. The process comprises the steps of laminating an anode layer, a separator/electrolyte layer, and a cathode layer, wherein the anode layer contains a continuous sheet or film of the integral graphene foam produced by the presently invented process. The continuous sheet or film of the graphene foam is pre-loaded with a liquid or gel electrolyte prior to being laminated to form a lithium metal battery sheet. Such a sheet-like battery can be rolled up, twisted, or folded back and forth to make many unique shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart illustrating various prior art processes of producing exfoliated graphite products (flexible graphite foils and expanded graphite flakes), along with a process for producing pristine graphene foam 40 a or graphene oxide foams 40 b;

FIG. 2 Schematic of a prior art lithium metal battery cell.

FIG. 3(A) Schematic of a graphene foam structure (cellular structure) having pores and graphene pore walls (cell walls) according to certain embodiments of the present disclosure.

FIG. 3(B) Schematic of a presently disclosed lithium metal battery cell (upper diagram) containing an anode current collector (e.g. Cu foil) but no anode active material (no lithium metal when the cell is manufactured or in a fully discharged state), a compressible graphene foam-based anode-protecting layer, a porous separator, and a cathode active material layer, which is typically composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector supporting the cathode active layer is also shown. The lower diagram shows the graphene foam being now deposited with or impregnated by lithium metal when the battery is in a charged state.

FIG. 3 (C) Schematic of a graphene foam structure (cellular structure) having pores and graphene pore walls (cell walls) according to certain embodiments of the present disclosure wherein the graphene foam further comprises a polymer.

FIG. 4 The cycling behaviors of (i) first cell containing nitrogen-doped graphene foam; (ii) a second cell containing graphene foam with no nitrogen doping; and (iii) a third cell containing no graphene foam as a protective layer in the anode.

FIG. 5 The battery cell capacity decay curves of three sodium metal cells: (a) one cell containing a pristine graphene foam and a sheet of Na foil as the anode active material, and NaFePO₄ as the cathode active material wherein an elastic polymer layer was disposed between the Na foil and the electrolyte; (b) a second sodium metal cell containing pristine graphene foam (but no elastic polymer protective layer) and a sheet of Na foil as the anode active material; and (c) a third cell contains a Na foil, but no graphene foam and no elastic polymer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As schematically illustrated in FIG. 2, a prior art lithium metal cell is typically composed of an anode current collector 202 (e.g. Cu foil), an anode active material layer 204 (a foil of lithium metal or lithium-rich metal alloy), a porous separator 230, a cathode active material layer 208 (containing a cathode active material, such as V₂O₅ and MoS₂ particles 234, and conductive additives that are all bonded by a resin binder, not shown), a cathode current collector 206 (e.g. Al foil), and an electrolyte disposed in ionic contact with both the anode active material layer 204 (also simply referred to as the “anode layer”) and the cathode active material layer 208 (or simply “cathode layer”). The entire cell is encased in a protective housing, such as a thin plastic-aluminum foil laminate-based envelop. A prior art sodium metal cell is similarly configured, but the anode active material layer is a foil of sodium metal or sodium-rich metal, or particles of sodium.

The prior art lithium or sodium metal cell is typically made by a process that includes the following steps: (a) The first step is mixing and dispersing particles of the cathode active material (e.g. lithium transition metal oxide particles), a conductive filler (e.g. acetylene black), a resin binder (e.g. PVDF) in a solvent (e.g. NMP) to form a cathode slurry; (b) The second step includes coating the cathode slurry on the surface(s) of an Al foil and drying the slurry to form a dried cathode electrode coated on the Al foil; (c) The third step includes laminating a Cu foil (as an anode current collector), a sheet of Li or Na foil (or lithium alloy or sodium alloy foil), a porous separator layer, and a cathode electrode-coated Al foil sheet together to form a 5-layer assembly, which is cut and slit into desired sizes and stacked to form a rectangular structure (as an example of shape) or rolled into a cylindrical cell structure; (d) The rectangular or cylindrical laminated structure is then encased in an aluminum-plastic laminated envelope or steel casing; and (e) A liquid electrolyte is then injected into the laminated structure to make a lithium battery cell.

Due to the high specific capacity of lithium metal and sodium metal, the highest battery energy density can be achieved by alkali metal rechargeable batteries that utilize a lithium metal or sodium metal as the anode active material, provided that a solution to the safety problem can be formulated. These cells include (a) the traditional Li or Na metal battery having a Li insertion or Na insertion compound in the cathode, (b) the Li-air or Na—O₂ cell that uses oxygen as a cathode instead of metal oxide (and Li or sodium metal as an anode instead of graphite or hard carbon), (c) the Li-sulfur or Na—S cell, and (d) the lithium-selenium cell or sodium-selenium cell.

The Li—O₂ battery is possibly the highest energy density electrochemical cell that can be configured today. The Li—O₂ cell has a theoretic energy density of 5,200 Wh/kg when oxygen mass is accounted for. A well configured Li—O₂ battery can achieve an energy density of 3,000 Wh/kg, which is 15-20 times greater than those of Li-ion batteries. However, current Li—O₂ batteries still suffer from poor energy efficiency, poor cycle efficiency, and dendrite formation issues. In the Li—S cell, elemental sulfur (S) as a cathode material exhibits a high theoretical Li storage capacity of 1,672 mAh/g. With a Li metal anode, the Li—S battery has a theoretical energy density of ˜1,600 Wh/kg. Despite its great potential, the practical realization of the Li—S battery has been hindered by several obstacles, such as low utilization of active material, high internal resistance, self-discharge, and rapid capacity fading on cycling. These technical barriers are due to the poor electrical conductivity of elemental sulfur, the high solubility of lithium polysulfides in organic electrolyte, the formation of inactivated Li₂S, and the formation of Li dendrites on the anode. Despite great efforts worldwide, dendrite formation remains the single most critical scientific and technological barrier against widespread implementation of all kinds of high energy density batteries having a Li metal anode.

We have discovered a highly dendrite-resistant, graphene foam-enabled Li metal cell or Na metal cell configuration that exhibits a high energy and/or high power density. Each cell contains a graphene foam as an anode protective layer. This graphene foam is composed of cell walls, comprising graphene sheets, and pores (e.g. 68 a and 68 b in FIG. 3(A)) to accommodate lithium or sodium.

FIG. 3(B) schematically shows certain embodiments of the presently disclosed lithium metal cell. The “anode-less” lithium metal battery cell (upper diagram) contains an anode current collector (e.g. Cu foil) but no anode active material (i.e. no lithium metal, when the cell is manufactured or in a fully discharged state), a compressible graphene foam-based anode-protecting layer, a porous separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector supporting the cathode active layer is also shown.

Such a battery cell having an initially lithium metal-free and sodium-free anode is commonly referred to as an “anode-less” lithium battery or sodium battery. For illustration purpose and using an anode-less lithium battery as an example, the lithium ions that are required for shuttling back and forth between the anode and the cathode are initially stored in the cathode active materials (e.g. Li in LiMn₂O₄ and LiMPO₄, where n=Ni, Co, F, Mn, etc.). During the first battery charge procedure, lithium ions (Li⁺) come out of the cathode active material, move through the electrolyte and then get deposited on a surface of the graphene foam and/or the anode current collector. As this charging procedure continues, more lithium ions get deposited onto the current collector surface or into pores of the graphene foam, eventually forming a lithium metal film or coating. The lower diagram of FIG. 3(B) shows the graphene foam is now deposited with or impregnated by lithium metal when the battery is in a charged state.

In certain other embodiments, this graphene foam can be lithiated (loaded with Li; e.g. Li⁺ ions permeated into pores to deposit on pore wall surfaces) or sodiated (loaded with Na) before or after the cell is made. For instance, when the cell is made, a foil or particles of lithium or sodium metal (or metal alloy) may be implemented at the anode (e.g. between the integral foam layer and the porous separator) to supply this foam with lithium or sodium. During the first battery discharge cycle, lithium (or sodium) is ionized, supplying lithium (or sodium) ions (Li⁺ or Na⁺) into electrolyte. These Li⁺ or Na⁺ ions migrate to the cathode side and get captured by and stored in the cathode active material (e.g. vanadium oxide, MoS₂, S, etc.).

During the subsequent re-charge cycle of the battery, Li⁺ or Na⁺ ions are released by the cathode active material and migrate back to the anode. These Li⁺ or Na⁺ ions naturally diffuse through the electrolyte and reach pore walls of the graphene foam. In this manner, the foam is said to be lithiated or sodiated. Alternatively, the graphene foam can be lithiated or sodiated (herein referred to as “pre-lithiated” or “pre-sodiated”) electrochemically prior to being incorporated as an anode protective layer into the cell structure. This can be accomplished by bringing a graphene foam layer in contact with a lithium or sodium foil in the presence of a liquid electrolyte, or by implementing a graphene foam layer as a working electrode and a lithium/sodium foil or rod as a counter-electrode in an electrochemical reactor chamber containing a liquid electrolyte. By introducing an electric current between the working electrode and the counter-electrode, one can introduce lithium or sodium into the foam, wherein Li⁺ or Na⁺ ions diffuse into the pores of the foam to form a lithium or sodium coating on pore walls.

We have discovered that this protective layer provides several unexpected benefits: (a) the formation of dendrite has been essentially eliminated; (b) uniform deposition of lithium back to the anode side is readily achieved; (c) the layer ensures smooth and uninterrupted transport of lithium ions from/to the anode current collector surface (or the lithium film deposited thereon during the battery operations) and through the interface between the current collector (or the lithium film deposited thereon) and the protective layer with minimal interfacial resistance; and (d) cycle stability can be significantly improved and cycle life increased.

Graphene is a single-atom thick layer of sp² carbon atoms arranged in a honeycomb-like lattice. Graphene can be readily prepared from graphite, activated carbon, graphite fibers, carbon black, and meso-phase carbon beads. Single-layer graphene and its slightly oxidized version (GO) can have a specific surface area (SSA) as high as 2670 m²/g. It is this high surface area that dramatically reduces the effective electrode current density, which in turn significantly reduces or eliminates the possibility of Li dendrite formation.

There is no limitation on the methods or processes for producing graphene foams. For instance, there are three common approaches: (a) a catalytic CVD process (followed by acid etching of Ni foil); (b) hydrothermal reduction of GO hydrogel; and (c) the use of a sacrificial material (e.g. colloidal polystyrene particles, PS) that is coated with graphene oxide sheets using a self-assembly approach (followed by dissolving PS).

The present disclosure provides a preferred reversibly compressible graphene foam composed of multiple pores and pore walls and a preferred process for producing such a foam. The pores in the graphene foam are formed slightly before, during, or after sheets of a graphene material are (1) chemically linked/merged together (edge-to-edge and/or face-to-face) typically at a temperature from 100 to 1,500° C. and/or (2) re-organized into larger graphite crystals or domains (herein referred to as re-graphitization) along the pore walls at a high temperature (typically >2,100° C. and more typically >2,500° C.). The process comprises:

(a) preparing a graphene dispersion having a graphene material dispersed in a liquid medium, wherein the graphene material is selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, or a combination thereof and wherein the dispersion contains an optional blowing agent with a blowing agent-to-graphene material weight ratio from 0/1.0 to 1.0/1.0 (this blowing agent is normally required if the graphene material is pristine graphene, typically having a blowing agent-to-pristine graphene weight ratio from 0.01/1.0 to 1.0/1.0);

(b) dispensing and depositing the graphene dispersion onto a surface of a supporting substrate (e.g. plastic film, rubber sheet, metal foil, glass sheet, paper sheet, etc.) to form a first wet layer of graphene material, wherein the dispensing and depositing procedure (e.g. coating or casting) includes subjecting the graphene dispersion to an orientation-inducing stress;

(c) partially or completely removing the liquid medium from the first wet layer of graphene material to form a first dried layer of graphene material having a content of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.) no less than 5% by weight (this non-carbon content, when being removed via heat-induced decomposition, produces volatile gases that act as a foaming agent or blowing agent);

(d) heat treating the first layer of graphene material at a first heat treatment temperature from 100° C. to 3,000° C. at a desired heating rate sufficient to induce volatile gas molecules from the non-carbon elements or to activate said blowing agent for producing the solid graphene foam. The graphene foam typically has a density from 0.01 to 1.7 g/cm³ (more typically from 0.1 to 1.5 g/cm³, and even more typically from 0.1 to 1.0 g/cm³, and most typically from 0.2 to 0.75 g/cm³), or a specific surface area from 50 to 3,000 m²/g (more typically from 200 to 2,000 m²/g, and most typically from 500 to 1,500 m²/g); and

(e) forming the solid graphene foam into a shape of a protective layer for a lithium or sodium metal battery. Such a shape may be a plate, a foil, a sheet, a continuous film, a fiber, a rod, a pipe, a hollow structure, etc.

A blowing agent or foaming agent is a substance which is capable of producing a cellular or foamed structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers (plastics and rubbers), glass, and metals. They are typically applied when the material being foamed is in a liquid state. It has not been previously known that a blowing agent can be used to create a foamed material while in a solid state. More significantly, it has not been taught or hinted that an aggregate of sheets of a graphene material can be converted into a graphene foam via a blowing agent. The cellular structure in a matrix is typically created for the purpose of reducing density, increasing thermal resistance and acoustic insulation, while increasing the thickness and relative stiffness of the original polymer.

Blowing agents or related foaming mechanisms to create pores or cells (bubbles) in a matrix for producing a foamed or cellular material, can be classified into the following groups:

-   -   (a) Physical blowing agents: e.g. hydrocarbons (e.g. pentane,         isopentane, cyclopentane), chlorofluorocarbons (CFCs),         hydrochlorofluorocarbons (HCFCs), and liquid CO₂. The         bubble/foam-producing process is endothermic, i.e. it needs heat         (e.g. from a melt process or the chemical exotherm due to         cross-linking), to volatize a liquid blowing agent.     -   (b) Chemical blowing agents: e.g. isocyanate, azo-, hydrazine         and other nitrogen-based materials (for thermoplastic and         elastomeric foams), sodium bicarbonate (e.g. baking soda, used         in thermoplastic foams). Here gaseous products and other         by-products are formed by a chemical reaction, promoted by         process or a reacting polymer's exothermic heat. Since the         blowing reaction involves forming low molecular weight compounds         that act as the blowing gas, additional exothermic heat is also         released. Powdered titanium hydride is used as a foaming agent         in the production of metal foams, as it decomposes to form         titanium and hydrogen gas at elevated temperatures.         Zirconium (II) hydride is used for the same purpose. Once formed         the low molecular weight compounds will never revert to the         original blowing agent(s), i.e. the reaction is irreversible.     -   (c) Mixed physical/chemical blowing agents: e.g. used to produce         flexible polyurethane (PU) foams with very low densities. Both         the chemical and physical blowing can be used in tandem to         balance each other out with respect to thermal energy         released/absorbed;

hence, minimizing temperature rise. For instance, isocyanate and water (which react to form CO₂) are used in combination with liquid CO₂ (which boils to give gaseous form) in the production of very low density flexible PU foams for mattresses.

-   -   (d) Mechanically injected agents: Mechanically made foams         involve methods of introducing bubbles into liquid polymerizable         matrices (e.g. an unvulcanized elastomer in the form of a liquid         latex). Methods include whisking-in air or other gases or low         boiling volatile liquids in low viscosity lattices, or the         injection of a gas into an extruder barrel or a die, or into         injection molding barrels or nozzles and allowing the shear/mix         action of the screw to disperse the gas uniformly to form very         fine bubbles or a solution of gas in the melt. When the melt is         molded or extruded and the part is at atmospheric pressure, the         gas comes out of solution expanding the polymer melt immediately         before solidification.     -   (e) Soluble and leachable agents: Soluble fillers, e.g. solid         sodium chloride crystals mixed into a liquid urethane system,         which is then shaped into a solid polymer part, the sodium         chloride is later washed out by immersing the solid molded part         in water for some time, to leave small inter-connected holes in         relatively high density polymer products.     -   (f) We have found that the above five mechanisms can all be used         to create pores in the graphene materials while they are in a         solid state. Another mechanism of producing pores in a graphene         material is through the generation and vaporization of volatile         gases by removing those non-carbon elements in a         high-temperature environment. This is a unique self-foaming         process that has never been previously taught or suggested.

In a preferred embodiment, the graphene material in the dispersion is selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene (e.g. nitrogen-doped graphene), chemically functionalized graphene, or a combination thereof. The starting graphitic material for producing any one of the above graphene materials may be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof. Production methods for these graphene sheets are well-known in the art.

For instance, the graphene oxide (GO) may be obtained by immersing powders or filaments of a starting graphitic material (e.g. natural graphite powder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid, nitric acid, and potassium permanganate) in a reaction vessel at a desired temperature for a period of time (typically from 0.5 to 96 hours, depending upon the nature of the starting material and the type of oxidizing agent used). The resulting graphite oxide particles may then be subjected to thermal exfoliation or ultrasonic wave-induced exfoliation to produce GO sheets.

Pristine graphene may be produced by direct ultrasonication (also known as liquid phase production) or supercritical fluid exfoliation of graphite particles. These processes are well-known in the art. Multiple pristine graphene sheets may be dispersed in water or other liquid medium with the assistance of a surfactant to form a suspension. A chemical blowing agent may then be dispersed into the dispersion (38 in FIG. 1). This suspension is then cast or coated onto the surface of a solid substrate (e.g. glass sheet or Al foil). When heated to a desired temperature, the chemical blowing agent is activated or decomposed to generate volatile gases (e.g. N₂ or CO₂), which act to form bubbles or pores in an otherwise mass of solid graphene sheets, forming a pristine graphene foam 40 a.

Fluorinated graphene or graphene fluoride is herein used as an example of the halogenated graphene material group. There are two different approaches that have been followed to produce fluorinated graphene: (1) fluorination of pre-synthesized graphene: This approach entails treating graphene prepared by mechanical exfoliation or by CVD growth with fluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation of multilayered graphite fluorides: Both mechanical exfoliation and liquid phase exfoliation of graphite fluoride can be readily accomplished [F. Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].

Interaction of F₂ with graphite at high temperature leads to covalent graphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperatures graphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n) carbon atoms are sp3-hybridized and thus the fluorocarbon layers are corrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n) only half of the C atoms are fluorinated and every pair of the adjacent carbon sheets are linked together by covalent C—C bonds. Systematic studies on the fluorination reaction showed that the resulting F/C ratio is largely dependent on the fluorination temperature, the partial pressure of the fluorine in the fluorinating gas, and physical characteristics of the graphite precursor, including the degree of graphitization, particle size, and specific surface area. In addition to fluorine (F₂), other fluorinating agents may be used, although most of the available literature involves fluorination with F₂ gas, sometimes in presence of fluorides.

For exfoliating a layered precursor material to the state of individual layers or few-layers, it is necessary to overcome the attractive forces between adjacent layers and to further stabilize the layers. This may be achieved by either covalent modification of the graphene surface by functional groups or by non-covalent modification using specific solvents, surfactants, polymers, or donor-acceptor aromatic molecules. The process of liquid phase exfoliation includes ultra-sonic treatment of a graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphene material, such as graphene oxide, to ammonia at high temperatures (200-400° C.). Nitrogenated graphene could also be formed at lower temperatures by a hydrothermal method; e.g. by sealing GO and ammonia in an autoclave and then increased the temperature to 150-250° C. Other methods to synthesize nitrogen doped graphene include nitrogen plasma treatment on graphene, arc-discharge between graphite electrodes in the presence of ammonia, ammonolysis of graphene oxide under CVD conditions, and hydrothermal treatment of graphene oxide and urea at different temperatures.

The pore walls (cell walls) in the presently invented graphene foam contain chemically bonded and merged graphene planes. These planar aromatic molecules or graphene planes (hexagonal structured carbon atoms) are well interconnected physically and chemically. The lateral dimensions (length or width) of these planes are huge (from 20 nm to >10 μm), typically several times or even orders of magnitude larger than the maximum crystallite dimension (or maximum constituent graphene plane dimension) of the starting graphite particles. The graphene sheets or planes are essentially interconnected to form electron-conducting pathways with low resistance. This is a unique and new class of material that has not been previously discovered, developed, or suggested to possibly exist.

In order to illustrate how the presently invented process works to produce a graphene foam, we herein make use of graphene oxide (GO) and graphene fluoride (GF) as two examples. These should not be construed as limiting the scope of our claims. In each case, the first step involves preparation of a graphene dispersion (e.g. GO+water or GF+organic solvent, DMF) containing an optional blowing agent. If the graphene material is pristine graphene containing no non-carbon elements, a blowing agent is required.

In step (b), the GF or GO suspension (21 in FIG. 1) is formed into a wet GF or GO layer 35 on a solid substrate surface (e.g. PET film or glass) preferably under the influence of a shear stress. One example of such a shearing procedure is casting or coating a thin film of GF or GO suspension using a coating machine. This procedure is similar to a layer of varnish, paint, coating, or ink being coated onto a solid substrate. The roller, “doctor's blade”, or wiper creates a shear stress when the film is shaped, or when there is a relative motion between the roller/blade/wiper and the supporting substrate. Quite unexpectedly and significantly, such a shearing action enables the planar GF or GO sheets to well align along, for instance, a shearing direction. Further surprisingly, such a molecular alignment state or preferred orientation is not disrupted when the liquid components in the GF or GO suspension are subsequently removed to form a well-packed layer of highly aligned GF or GO sheets that are at least partially dried. The dried GF or GO mass 37 a has a high birefringence coefficient between an in-plane direction and the normal-to-plane direction.

In an embodiment, this GF or GO layer is then subjected to a heat treatment to activate the blowing agent and/or the thermally-induced reactions that remove the non-carbon elements (e.g. F, O, etc.) from the graphene sheets to generate volatile gases as by-products. These volatile gases generate pores or bubbles inside the solid graphene material, pushing solid graphene sheets into a wall structure, forming a graphene oxide foam 40 b. If no blowing agent is added, the non-carbon elements in the graphene material preferably occupy at least 10% by weight of the graphene material (preferably at least 20%, and further preferably at least 30%). The first (initial) heat treatment temperature is typically greater than 80° C., preferably greater than 100° C., more preferably greater than 300° C., further more preferably greater than 500° C. and can be as high as 1,500° C. The blowing agent is typically activated at a temperature from 80° C. to 300° C., but can be higher. The foaming procedure (formation of pores, cells, or bubbles) is typically completed within the temperature range of 80-1,500° C. Quite surprisingly, the chemical linking or merging between graphene planes (GO or GF planes) in an edge-to-edge and face-to-face manner can occur at a relatively low heat treatment temperature (e.g. as low as from 150 to 300° C.).

The foamed graphene material may be subjected to a further heat treatment that involves at least a second temperature that is significantly higher than the first heat treatment temperature.

A properly programmed heat treatment procedure can involve just a single heat treatment temperature (e.g. a first heat treatment temperature only), at least two heat treatment temperatures (first temperature for a period of time and then raised to a second temperature and maintained at this second temperature for another period of time), or any other combination of heat treatment temperatures (HTT) that involve an initial treatment temperature (first temperature) and a final HTT (second), higher than the first. The highest or final HTT that the dried graphene layer experiences may be divided into four distinct HTT regimes:

-   Regime 1 (80° C. to 300° C.): In this temperature range (the thermal     reduction regime and also the activation regime for a blowing agent,     if present), a GO or GF layer primarily undergoes thermally-induced     reduction reactions, leading to a reduction of oxygen content or     fluorine content from typically 20-50% (of 0 in GO) or 10-25% (of F     in GF) to approximately 5-6%. This treatment results in a reduction     of inter-graphene spacing in foam walls from approximately 0.6-1.2     nm (as dried) down to approximately 0.4 nm, and an increase in     thermal conductivity to 200 W/mK per unit specific gravity and/or     electrical conductivity to 2,000 S/cm per unit of specific gravity.     (Since one can vary the level of porosity and, hence, specific     gravity of a graphene foam material and, given the same graphene     material, both the thermal conductivity and electric conductivity     values vary with the specific gravity, these property values must be     divided by the specific gravity to facilitate a fair comparison.)     Even with such a low temperature range, some chemical linking     between graphene sheets occurs. The inter-GO or inter-GF planar     spacing remains relatively large (0.4 nm or larger). Many O- or     F-containing functional groups survive. -   Regime 2 (300° C-1,500° C.): In this chemical linking regime,     extensive chemical combination, polymerization, and cross-linking     between adjacent GO or GF sheets occur. The oxygen or fluorine     content is reduced to typically <1.0% (e.g. 0.7%) after chemical     linking, resulting in a reduction of inter-graphene spacing to     approximately 0.345 nm. This implies that some initial     re-graphitization has already begun at such a low temperature, in     stark contrast to conventional graphitizable materials (such as     carbonized polyimide film) that typically require a temperature as     high as 2,500° C. to initiate graphitization. This is another     distinct feature of the presently invented graphene foam and its     production processes. These chemical linking reactions result in an     increase in thermal conductivity to 250 W/mK per unit of specific     gravity, and/or electrical conductivity to 2,500-4,000 S/cm per unit     of specific gravity. -   Regime 3 (1,500-2,500° C.): In this ordering and re-graphitization     regime, extensive graphitization or graphene plane merging occurs,     leading to significantly improved degree of structural ordering in     the foam walls. As a result, the oxygen or fluorine content is     reduced to typically 0.01% and the inter-graphene spacing to     approximately 0.337 nm (achieving degree of graphitization from 1%     to approximately 80%, depending upon the actual HTT and length of     time). The improved degree of ordering is also reflected by an     increase in thermal conductivity to >350 W/mK per unit of specific     gravity, and/or electrical conductivity to >3,500 S/cm per unit of     specific gravity. -   Regime 4 (higher than 2,500° C.): In this re-crystallization and     perfection regime, extensive movement and elimination of grain     boundaries and other defects occur, resulting in the formation of     nearly perfect single crystals or poly-crystalline graphene crystals     with huge grains in the foam walls, which can be orders of magnitude     larger than the original grain sizes of the starting graphite     particles for the production of GO or GF. The oxygen or fluorine     content is essentially eliminated, typically 0%-0.001%. The     inter-graphene spacing is reduced to down to approximately 0.3354 nm     (degree of graphitization from 80% to nearly 100%), corresponding to     that of a perfect graphite single crystal. The foamed structure thus     obtained exhibits a thermal conductivity of >400 W/mK per unit of     specific gravity, and electrical conductivity of >4,000 S/cm per     unit of specific gravity.

The presently invented graphene foam structure can be obtained by heat-treating the dried GO or GF layer with a temperature program that covers at least the first regime (typically requiring 1-4 hours in this temperature range if the temperature never exceeds 500° C.), more commonly covers the first two regimes (1-2 hours preferred), still more commonly the first three regimes (preferably 0.5-2.0 hours in Regime 3), and can cover all the 4 regimes (including Regime 4 for 0.2 to 1 hour, may be implemented to achieve the highest conductivity).

If the graphene material is selected from the group of non-pristine graphene materials consisting of graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, or a combination thereof, and wherein the maximum heat treatment temperature (e.g. both the first and second heat treatment temperatures) is (are) less than 2,500° C., then the resulting solid graphene foam typically contains a content of non-carbon elements in the range from 0.01% to 2.0% by weight (non-pristine graphene foam).

X-ray diffraction patterns were obtained with an X-ray diffractometer equipped with CuKcv radiation. The shift and broadening of diffraction peaks were calibrated using a silicon powder standard. The degree of graphitization, g, was calculated from the X-ray pattern using the Mering's Eq, d₀₀₂=0.3354 g+0.344 (1-g), where d₀₀₂ is the interlayer spacing of graphite or graphene crystal in nm. This equation is valid only when d₀₀₂ is equal or less than approximately 0.3440 nm. The graphene foam walls having a d₀₀₂ higher than 0.3440 nm reflects the presence of oxygen- or fluorine-containing functional groups (such as —F, —OH, >O, and —COOH on graphene molecular plane surfaces or edges) that act as a spacer to increase the inter-graphene spacing.

Another structural index that can be used to characterize the degree of ordering of the stacked and bonded graphene planes in the foam walls of graphene and conventional graphite crystals is the “mosaic spread,” which is expressed by the full width at half maximum of a rocking curve (X-ray diffraction intensity) of the (002) or (004) reflection. This degree of ordering characterizes the graphite or graphene crystal size (or grain size), amounts of grain boundaries and other defects, and the degree of preferred grain orientation. A nearly perfect single crystal of graphite is characterized by having a mosaic spread value of 0.2-0.4. Most of our graphene walls have a mosaic spread value in this range of 0.2-0.4 (if produced with a heat treatment temperature (HTT) no less than 2,500° C.). However, some values are in the range from 0.4-0.7 if the HTT is between 1,500 and 2,500° C., and in the range from 0.7-1.0 if the HTT is between 300 and 1,500° C.

This graphene foam wall is not made by gluing or bonding discrete flakes/platelets together with a resin binder, linker, or adhesive. Instead, GO sheets (molecules) from the GO dispersion or the GF sheets from the GF dispersion are merged through joining or forming of covalent bonds with one another, into an integrated graphene entity, without using any externally added linker or binder molecules or polymers.

This graphene foam wall is typically a poly-crystal composed of large grains having incomplete grain boundaries. This entity is derived from a GO or GF suspension, which is in turn obtained from natural graphite or artificial graphite particles originally having multiple graphite crystallites. Prior to being chemically oxidized or fluorinated, these starting graphite crystallites have an initial length (L_(a) in the crystallographic a-axis direction), initial width (L_(b) in the b-axis direction), and thickness (L_(c) in the c-axis direction). Upon oxidation or fluorination, these initially discrete graphite particles are chemically transformed into highly aromatic graphene oxide or graphene fluoride molecules having a significant concentration of edge- or surface-borne functional groups (e.g. —F, —OH, —COOH, etc.). These aromatic GO or GF molecules in the suspension have lost their original identity of being part of a graphite particle or flake. Upon removal of the liquid component from the suspension, the resulting GO or GF molecules form an essentially amorphous structure. Upon heat treatments, these GO or GF molecules are chemically merged and linked into a unitary or monolithic graphene entity that constitutes the foam wall. This foam wall is highly ordered.

The resulting unitary graphene entity in the foam wall typically has a length or width significantly greater than the L_(a) and L_(b) of the original crystallites. The length/width of this graphene foam wall entity is significantly greater than the L_(a) and L_(b) of the original crystallites. Even the individual grains in a poly-crystalline graphene wall structure have a length or width significantly greater than the L_(a) and L_(b) of the original crystallites.

Due to these unique chemical composition (including oxygen or fluorine content), morphology, crystal structure (including inter-graphene spacing), and structural features (e.g. high degree of orientations, few defects, incomplete grain boundaries, chemical bonding and no gap between graphene sheets, and substantially no interruptions in graphene planes), the GO- or GF-derived graphene foam has a unique combination of outstanding thermal conductivity, electrical conductivity, mechanical strength, and stiffness (elastic modulus).

In certain embodiments, in the alkali metal battery, the graphene foam further comprises a polymer that impregnates into the pores or is coated on or bonded to surfaces of the pore walls, wherein the polymer comprises an elastomer, an elastic polymer, an electron-conducting polymer, an ion-conducting polymer, or a combination thereof. FIG. 3(C) shows an embodiment where the graphene foam further comprises a polymer 70 that impregnates into the pores or is coated on or bonded to surfaces of the pore walls.

Most of these polymers can be initially in a monomer state that is in a liquid form or can be made into a liquid or solution state (e.g. via melting or dissolution in a liquid solvent). The monomer can be mixed with an initiator and an optional crosslinking agent to form a reacting mass, which is deposited into the pores or onto the pore wall surfaces of the graphene foam, followed by polymerization and crosslinking, where appropriate. These polymers can be in a non-crosslinked form that is soluble in a liquid solvent. The liquid solution containing a polymer dissolved in a liquid solvent and a curing agent (or crosslinking agent), also dissolved or dispersed in the solution, can also be deposited onto the graphene foam cell wall surfaces or into the pores of the cellular (foamed) structure, followed by removal of the liquid solvent and necessary crosslinking via heat or UV curing. Deposition of the reactive polymer or polymer solution may be conducted using spraying, spray-coating, coating, casting, solution immersion (dipping), etc.

The elastomer preferably comprises a polymer selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, a copolymer thereof, a sulfonated version thereof, or a combination thereof

The elastic polymer preferably contains a cross-linked network of polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.

For instance, ethoxylated trimethylopropane triacrylate monomer (ETPTA, Mw=428, chemical formula given below), along with an initiator, can be dissolved in an organic solvent, such as ethylene carbonate (EC) or diethyl carbonate (DEC). Then, the ETPTA monomer/solvent/initiator solution may be cast to form ETPTA a monomer/initiator layer on a glass surface. The layer can then be thermally cured to obtain a thin layer of a high-elasticity polymer. The polymerization and cross-linking reactions of this monomer can be initiated by a radical initiator derived from benzoyl peroxide (BPO) or AIBN through thermal decomposition of the initiator molecule. The ETPTA monomer has the following chemical formula:

As another example, the high-elasticity polymer for anode lithium foil/coating protection may be based on cationic polymerization and cross-linking of the cyanoethyl polyvinyl alcohol (PVA-CN, Formula 2) in succinonitrile (SN).

The procedure may begin with dissolving PVA-CN in succinonitrile (NCCH₂CH₂CN) to form a mixture solution. This is followed by adding an initiator into the mixture solution. For instance, LiPF₆ can be added into the PVA-CN/SN mixture solution at a weight ratio (selected from the preferred range from 20:1 to 2:1) to form a precursor solution. Then, the solution may be deposited to form a thin layer of reacting mass, PVA-CN/LiPF₆, which is subsequently heated at a temperature (e.g. from 75 to 100° C.) for 2 to 8 hours to obtain a high-elasticity polymer.

During this process, cationic polymerization and cross-linking of cyano groups on the PVA-CN may be initiated by PF₅, which is derived from the thermal decomposition of LiPF₆ at such an elevated temperature.

It is essential or advantageous for these materials to form a lightly cross-linked network of polymer chains. In other words, the network polymer or cross-linked polymer should have a relatively low degree of cross-linking or low cross-link density to impart a high elastic deformation.

The ion-conducting polymer is preferably selected from poly(ethylene oxide) (PEO), polypropylene oxide (PPO), poly(ethylene glycol) (PEG), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), cyanoethyl poly(vinyl alcohol) (PVACN), polyacrylic acid (PAA), a pentaerythritol tetraacrylate (PETEA)-based polymer, an aliphatic polycarbonate (including poly(vinylene carbonate) (PVC), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and poly(trimethylene carbonate) (PTMC)), a single Li-ion conducting solid polymer electrolyte with a carboxylate anion, a sulfonylimide anion, or sulfonate anion, a crosslinked electrolyte of poly(ethylene glycol) diacrylate (PEGDA) or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

The electron-conducting polymer preferably comprises chains of a conjugated polymer selected from polyacetylene, polythiophene, poly(3-alkylthiophenes), polypyrrole, polyaniline, poly(isothianaphthene), poly(3,4-ethylenedioxythiophene), alkoxy-substituted poly(p-phenylene vinylene), poly(2,5-bis(cholestanoxy) phenylene vinylene), poly(p-phenylene vinylene), poly(2,5-dialkoxy) paraphenylene vinylene, poly[(1,4-phenylene-1,2-diphenylvinylene)], poly(3′,7′-dimethyloctyloxy phenylene vinylene), polyparaphenylene, polyparaphenylene, polyparaphenylene sulfide, polyheptadiyne, poly(3-hexylthiophene), poly(3-octylthiophene), poly(3-cyclohexylthiophene), poly(3-methyl-4-cyclohexylthiophene), poly(2,5-dialkoxy-1,4-phenyleneethynylene), poly(2-decyloxy-1,4-phenylene), poly(9,9-dioctylfluorene), polyquinoline, a derivative thereof, a copolymer thereof, a sulfonated version thereof, or a combination thereof.

In certain embodiments, the elastomer, elastic polymer, electron-conducting polymer or ion-conducting polymer further contains a lithium salt dispersed therein (in the polymer) and the lithium salt is preferably selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃S0₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

Electrolyte is an important ingredient in a battery. A wide range of electrolytes can be used for practicing the instant disclosure. Most preferred are non-aqueous, polymer gel, and solid-state electrolytes although other types can be used. Polymer, polymer gel, and solid-state electrolytes are preferred over liquid electrolyte.

The non-aqueous electrolyte to be employed herein may be produced by dissolving an electrolytic salt in a non-aqueous solvent. Any known non-aqueous solvent which has been employed as a solvent for a lithium secondary battery can be employed. A non-aqueous solvent mainly consisting of a mixed solvent comprising ethylene carbonate (EC) and at least one kind of non-aqueous solvent whose melting point is lower than that of aforementioned ethylene carbonate and whose donor number is 18 or less (hereinafter referred to as a second solvent) may be preferably employed. This non-aqueous solvent is advantageous in that it is (a) effective in suppressing the reductive or oxidative decomposition of electrolyte; and (b) high in conductivity. A non-aqueous electrolyte solely composed of ethylene carbonate (EC) is advantageous in that it is relatively stable against carbonaceous filament materials. However, the melting point of EC is relatively high, 39 to 40° C., and the viscosity thereof is relatively high, so that the conductivity thereof is low, thus making EC alone unsuited for use as a secondary battery electrolyte to be operated at room temperature or lower. The second solvent to be used in a mixture with EC functions to make the viscosity of the solvent mixture lower than that of EC alone, thereby promoting the ion conductivity of the mixed solvent. Furthermore, when the second solvent having a donor number of 18 or less (the donor number of ethylene carbonate is 16.4) is employed, the aforementioned ethylene carbonate can be easily and selectively solvated with lithium ion, so that the reduction reaction of the second solvent with the carbonaceous material well developed in graphitization is assumed to be suppressed. Further, when the donor number of the second solvent is controlled to not more than 18, the oxidative decomposition potential to the lithium electrode can be easily increased to 4 V or more, so that it is possible to manufacture a lithium secondary battery of high voltage.

Preferable second solvents are dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene and methyl acetate (MA). These second solvents may be employed singly or in a combination of two or more. More desirably, this second solvent should be selected from those having a donor number of 16.5 or less. The viscosity of this second solvent should preferably be 28 cps or less at 25° C.

The electrolytic salts to be incorporated into a non-aqueous electrolyte may be selected from a lithium salt such as lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃) and bis-trifluoromethyl sulfonylimide lithium [LiN(CF₃SO₂)₂]. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ are preferred. The content of aforementioned electrolytic salts in the non-aqueous solvent is preferably 0.5 to 3.5 mol/l.

For sodium metal batteries, the organic electrolyte may contain an alkali metal salt preferably selected from sodium perchlorate (NaClO₄), potassium perchlorate (KClO₄), sodium hexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodium hexafluoroarsenide, potassium hexafluoroarsenide, sodium trifluoro-methanesulfonate (NaCF₃SO₃), potassium trifluoro-methanesulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂), bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), an ionic liquid salt, or a combination thereof.

The ionic liquid is composed of ions only. Ionic liquids are low melting temperature salts that are in a molten or liquid state when above a desired temperature. For instance, a salt is considered as an ionic liquid if its melting point is below 100° C. If the melting temperature is equal to or lower than room temperature (25° C.), the salt is referred to as a room temperature ionic liquid (RTIL). The IL salts are characterized by weak interactions, due to the combination of a large cation and a charge-delocalized anion. This results in a low tendency to crystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a 1-ethyl-3-methylimidazolium (EMI) cation and an N,N-bis(trifluoromethane)sulfonamide (TFSI) anion. This combination gives a fluid with an ionic conductivity comparable to many organic electrolyte solutions and a low decomposition propensity and low vapor pressure up to ˜300-400° C. This implies a generally low volatility and non-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in an essentially unlimited number of structural variations owing to the preparation ease of a large variety of their components. Thus, various kinds of salts can be used to design the ionic liquid that has the desired properties for a given application. These include, among others, imidazolium, pyrrolidinium and quaternary ammonium salts as cations and bis(trifluoromethanesulfonyl) imide, bis(fluorosulfonyl)imide, and hexafluorophosphate as anions. Based on their compositions, ionic liquids come in different classes that basically include aprotic, protic and zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, but not limited to, tetraalkylammonium, di-, tri-, and tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc. Relatively speaking, the combination of imidazolium- or sulfonium-based cations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs with good working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionic conductivity, high thermal stability, low volatility, low (practically zero) vapor pressure, non-flammability, the ability to remain liquid at a wide range of temperatures above and below room temperature, high polarity, high viscosity, and wide electrochemical windows. These properties, except for the high viscosity, are desirable attributes when it comes to using an RTIL as an electrolyte ingredient (a salt and/or a solvent) in a battery.

The cathode active material may be selected from a wide variety of oxides, such as lithium-containing nickel oxide, cobalt oxide, nickel-cobalt oxide, vanadium oxide, multiple transition metal oxides (e.g. well-known NCM and NCA) and lithium iron phosphate. These oxides may contain a dopant, which is typically a metal element or several metal elements. The cathode active material may also be selected from chalcogen compounds, such as titanium disulfate, molybdenum disulfate, and metal sulfides. More preferred are lithium cobalt oxide (e.g., Li_(x)CoO₂ where 0.8≤x≤1), lithium nickel oxide (e.g., LiNiO₂), lithium manganese oxide (e.g., LiMn₂O₄ and LiMnO₂), lithium iron phosphate, lithium manganese-iron phosphate, lithium vanadium phosphate, and the like. Sulfur or lithium polysulfide may also be used in a Li—S cell.

For an anode-less lithium metal battery or sodium metal battery, the cathode active material needs to provide the required lithium ions or sodium ions and, thus, it must be lithiated (e.g. containing the element Li in LiMn₂O₄) or sodiated (containing Na in the chemical formula, such as in NaFePO₄).

The rechargeable lithium metal batteries can make use of non-lithiated compounds, such as TiS₂, MoS₂, MnO₂, CoO₂, V₃O₈, and V₂O₅, as the cathode active materials. The lithium vanadium oxide may be selected from the group consisting of VO₂, Li_(x)VO₂, V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5. In general, the inorganic material-based cathode materials may be selected from a metal carbide, metal nitride, metal boride, metal dichalcogenide, or a combination thereof. Preferably, the desired metal oxide or inorganic material is selected from an oxide, dichalcogenide, trichalcogenide, sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese, iron, or nickel in a nanowire, nano-disc, nano-ribbon, or nano platelet form. These materials can be in the form of a simple mixture with sheets of a graphene material, but preferably in a nano particle or nano coating form that that is physically or chemically bonded to a surface of the graphene sheets.

Preferably, the cathode active material for a sodium metal battery contains a sodium intercalation compound or a potassium intercalation compound selected from NaFePO₄, KFePO₄, Na(_(1-x))K_(x)PO₄, Na_(0.7)FePO₄, Na_(1.5)VOPO₄F_(0.5), Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, Na₂FePO₄F, NaFeF₃, NaVPO₄F, KVPO₄F, Na₃V₂(PO₄)₂F₃, Na_(1.5)VOPO₄F_(0.5), Na₃V₂(PO₄)₃, NaV₆O₁₅, Na_(x)VO₂, Na_(0.33)V₂O₅, Na_(x)CoO₂, Na_(2/3)[Ni_(1/3)Mn_(2/3)]O₂, Na_(x)(Fe_(1/2)Mn_(1/2))O₂, Na_(x)MnO₂, Na_(x)K_((1-x))MnO₂, Na_(0.44)MnO₂, Na_(0.44)MnO₂/C, Na₄Mn₉O₁₈, NaFe₂Mn(PO₄)₃, Na₂Ti₃O₇, Ni_(1/3)Mn_(1/3)Co_(1/3)O₂, Cu_(0.56)Ni_(0.44)HCF, NiHCF, Na_(x)MnO₂, NaCrO₂, KCrO₂, Na₃Ti₂(PO₄)₃, NiCo₂O₄, Ni₃S₂/FeS₂, Sb₂O₄, Na₄Fe(CN)₆/C, NaV_(1-x)Cr_(x)PO₄F, Se_(z)S_(y) (y/z=0.01 to 100), Se, Alluaudites, or a combination thereof, wherein x is from 0.1 to 1.0.

The organic material or polymeric material-based cathode materials may be selected from Poly(anthraquinonyl sulfide) (PAQS), a lithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT), polymer-bound PYT, Quino(triazene), redox-active organic material, Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxy anthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n), lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer, Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile (HAT(CN)₆), 5-B enzylidene hydantoin, Isatine lithium salt, Pyromellitic diimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄), N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP), N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP), N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, a quinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT), 5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ), 5-amino-1,4-dyhydroxy anthraquinone (ADAM), calixquinone, Li₄C₆O₆, Li₂C₆O₆, Li₆C₆O₆, Na_(x)C₆O₆ (x=1−3), Na₂(C₆H₂O₄), Na₂C₈H₄O₄ (Na terephthalate), Na₂C₆H₄O₄(Na trans-trans-muconate), or a combination thereof.

The thioether polymer is selected from Poly[methanetetryl-tetra(thiomethylene)] (PMTTM), Poly(2,4-dithiopentanylene) (PDTP), a polymer containing Poly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioether polymers, a side-chain thioether polymer having a main-chain consisting of conjugating aromatic moieties, and having a thioether side chain as a pendant, Poly(2-phenyl-1,3 -dithiolane) (PPDT), Poly(1,4-di(1,3 -dithiolan-2-yl)benzene) (PDDTB), poly(tetrahydrobenzodithiophene) (PTHBDT), poly[1,2,4,5-tetrakis(propylthio) benzene] (PTKPTB, or poly[3,4(ethylenedithio)thiophene] (PEDTT).

The organic material that can be used as a cathode active material in a lithium metal battery or sodium metal battery may include a phthalocyanine compound selected from copper phthalocyanine, zinc phthalocyanine, tin phthalocyanine, iron phthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadyl phthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine, manganous phthalocyanine, dilithium phthalocyanine, aluminum phthalocyanine chloride, cadmium phthalocyanine, chlorogallium phthalocyanine, cobalt phthalocyanine, silver phthalocyanine, a metal-free phthalocyanine, a chemical derivative thereof, or a combination thereof.

The following examples are used to illustrate some specific details about the best modes of practicing the instant disclosure and should not be construed as limiting the scope of the disclosure.

Example 1: Various Blowing Agents and Pore-Forming (Bubble-Producing) Processes

In the field of plastic processing, chemical blowing agents are mixed into the plastic pellets in the form of powder or pellets and dissolved at higher temperatures. Above a certain temperature specific for blowing agent dissolution, a gaseous reaction product (usually nitrogen or CO₂) is generated, which acts as a blowing agent. However, a chemical blowing agent cannot be dissolved in a graphene material, which is a solid, not liquid. This presents a challenge to make use of a chemical blowing agent to generate pores or cells in a graphene material.

After extensive experimenting, we have discovered that practically any chemical blowing agent (e.g. in a powder or pellet form) can be used to create pores or bubbles in a dried layer of graphene when the first heat treatment temperature is sufficient to activate the blowing reaction. The chemical blowing agent (powder or pellets) may be dispersed in the liquid medium to become a second dispersed phase (sheets of graphene material being the first dispersed phase) in the suspension, which can be deposited onto the solid supporting substrate to form a wet layer. This wet layer of graphene material may then be dried and heat treated to activate the chemical blowing agent. After a chemical blowing agent is activated and bubbles are generated, the resulting foamed graphene structure is largely maintained even when subsequently a higher heat treatment temperature is applied to the structure. This is quite unexpected, indeed.

Chemical foaming agents (CFAs) can be organic or inorganic compounds that release gasses upon thermal decomposition. CFAs are typically used to obtain medium- to high-density foams, and are often used in conjunction with physical blowing agents to obtain low-density foams. CFAs can be categorized as either endothermic or exothermic, which refers to the type of decomposition they undergo. Endothermic types absorb energy and typically release carbon dioxide and moisture upon decomposition, while the exothermic types release energy and usually generate nitrogen when decomposed. The overall gas yield and pressure of gas released by exothermic foaming agents is often higher than that of endothermic types. Endothermic CFAs are generally known to decompose in the range from 130 to 230° C. (266-446° F.), while some of the more common exothermic foaming agents decompose around 200° C. (392° F.). However, the decomposition range of most exothermic CFAs can be reduced by addition of certain compounds. The activation (decomposition) temperatures of CFAs fall into the range of our heat treatment temperatures. Examples of suitable chemical blowing agents include sodium bi-carbonate (baking soda), hydrazine, hydrazide, azodicarbonamide (exothermic chemical blowing agents), nitroso compounds (e.g. N, N-Dinitroso pentamethylene tetramine), hydrazine derivatives (e.g. 4, 4′-Oxybis (benzenesulfonyl hydrazide) and Hydrazo dicarbonamide), and hydrogen carbonate (e.g. Sodium hydrogen carbonate). These are all commercially available in plastics industry.

In the production of foamed plastics, physical blowing agents are metered into the plastic melt during foam extrusion or injection molded foaming, or supplied to one of the precursor materials during polyurethane foaming. It has not been previously known that a physical blowing agent can be used to create pores in a graphene material, which is in a solid state (not melt). We have surprisingly observed that a physical blowing agent (e.g. CO₂ or N₂) can be injected into the stream of graphene suspension prior to being coated or cast onto the supporting substrate. This would result in a foamed structure even when the liquid medium (e.g. water and/or alcohol) is removed. The dried layer of graphene material is capable of maintaining a controlled amount of pores or bubbles during liquid removal and subsequent heat treatments.

Technically feasible blowing agents include Carbon dioxide (CO₂), Nitrogen (N₂), Isobutane (C₄H₁₀), Cyclopentane (C₅H₁₀), Isopentane (C₅H₁₂), CFC-11 (CFCI₃), HCFC-22 (CHF₂CI), HCFC-142b (CF₂CICH₃), and HCFC-134a (CH₂FCF₃). However, in selecting a blowing agent, environmental safety is a major factor to consider. The Montreal Protocol and its influence on consequential agreements pose a great challenge for the producers of foam. Despite the effective properties and easy handling of the formerly applied chlorofluorocarbons, there was a worldwide agreement to ban these because of their ozone depletion potential (ODP). Partially halogenated chlorofluorocarbons are also not environmentally safe and therefore already forbidden in many countries. The alternatives are hydrocarbons, such as isobutane and pentane, and the gases such as CO₂ and nitrogen.

Except for those regulated substances, all the blowing agents recited above have been tested in our experiments. For both physical blowing agents and chemical blowing agents, the blowing agent amount introduced into the suspension is defined as a blowing agent-to-graphene material weight ratio, which is typically from 0/1.0 to 1.0/1.0.

Example 2: Preparation of GO Foam and Nitrogen-Doped Graphene Foam

Natural graphite particles were used as a starting material, which was immersed in a mixture of concentrated sulfuric acid, nitric acid, and potassium permanganate (as the chemical intercalate and oxidizer) to prepare graphite intercalation compounds (GIC s). The starting material was first dried in a vacuum oven for 24 h at 80° C. Then, a mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (at a weight ratio of 4:1:0.05) was slowly added, under appropriate cooling and stirring, to a three-neck flask containing fiber segments. After 5-16 hours of reaction, the acid-treated natural graphite particles were filtered and washed thoroughly with deionized water until the pH level of the solution reached 5. After being dried at 60° C. overnight, the resulting graphite intercalation compound (GIC) or graphite oxide was re-dispersed in water and/or alcohol to form a slurry.

In one sample, five grams of the graphite oxide were mixed with 2,000 ml alcohol solution consisting of alcohol and distilled water with a ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry was subjected to ultrasonic irradiation with a power of 200 W for various lengths of time. After 20 minutes of sonication, GO particles were effectively exfoliated and separated into thin graphene oxide sheets with oxygen content of approximately 23%-47% by weight. The resulting suspension contains GO sheets being suspended in water. A chemical blowing agent (hydrazo dicarbonamide) was added to the suspension just prior to casting.

The resulting suspension was then cast onto a glass surface using a doctor's blade to exert shear stresses, inducing GO sheet orientations. The resulting GO coating films, after removal of liquid, have a thickness that can be varied from approximately 5 to 500 μm (preferably and typically from 10 μm to 50 μm).

For making a graphene foam specimen, the GO coating film was then subjected to heat treatments that typically involved an initial thermal reduction temperature of 80-350° C. for 1-8 hours, followed by heat-treating at a second temperature of 850-1,200° C. for 0.5 to 5 hours.

In one sample, an attempt was made to obtain nitrogen doped GO foam. Nitrogen-doped GO foam was obtained by simultaneously reducing the GO film while incorporating nitrogen in a high temperature ammonia atmosphere. GO films were placed in a ceramic crucible within a quartz furnace tube. The tube was first purged with Ar (100 mL flow) for 60 minutes and then ammonia was introduced at 60 sccm while temperature was increased at 850° C. at a rate of 1° C/min. The temperature was maintained at 850° C. for 3 hours and slowly lowered to room temperature.

The reversible compressibility of these graphene foam samples are typically in the range from 45% -90%.

In order to determine the relative stability of the graphene foam-protected lithium metal anode structure, the voltage profiles of symmetric layered Li-foam electrode cells (no nitrogen doping), symmetric layered Li-foam (nitrogen-doped) electrode cells, and the bare Li foil counterparts were obtained through over 200 cycles at nominal current density of 1 mA/cm² (foam specific surface area not taken into account, just plain electrode surface area). The symmetric layered Li—N-doped foam electrode cells exhibited stable voltage profiles with negligible hysteresis, whereas the bare Li foils displayed a rapid increase in hysteresis during cycling, by almost 100% after 100 cycles. The hysteresis growth rate of symmetric layered Li-foam (no nitrogen doping) electrode cells is significantly greater than that of symmetric layered Li—N-doped foam electrode cells, but lower than that of the bare Li foils. For symmetric layered Li-foam electrode cells, flat voltage plateau at both the charging and discharging states can be retained throughout the whole cycle without obvious increases in hysteresis. This is a significant improvement compared with bare Li electrodes, which showed fluctuating voltage profiles with consistently higher overpotential at both the initial and final stages of each stripping/plating process. After 300 cycles, there is no sign of dendrite formation and the lithium deposition is very even in symmetric layered Li—N-doped foam electrode cells. For the symmetric layered Li-foam (no nitrogen doping) electrode cells, some lithium tends to deposit unevenly on external surfaces of pores, instead of fully entering the pores. Typically, for bare Li foil electrodes, dendrite begins to develop in less than 30 cycles.

Example 3: Preparation of Pristine Graphene Foam (Substantially Oxygen-Free)

Recognizing the possibility of the high defect population in GO sheets acting to reduce the conductivity of individual graphene plane, we decided to study if the use of pristine graphene sheets (non-oxidized and oxygen-free, non-halogenated and halogen-free, etc.) can lead to a graphene foam having a higher thermal conductivity. Pristine graphene sheets were produced by using the direct ultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground to approximately 20 μm or less in sizes, were dispersed in 1,000 mL of deionized water (containing 0.1% by weight of a dispersing agent, Zonyl® FSO from DuPont) to obtain a suspension. An ultrasonic energy level of 85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation, and size reduction of graphene sheets for a period of 15 minutes to 2 hours. The resulting graphene sheets are pristine graphene that have never been oxidized and are oxygen-free and relatively defect-free. There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) of chemical bowing agents (N, N-Dinitroso pentamethylene tetramine or 4, 4′-Oxybis (benzenesulfonyl hydrazide) were added to a suspension containing pristine graphene sheets and a surfactant. The suspension was then cast onto a glass surface using a doctor's blade to exert shear stresses, inducing graphene sheet orientations. Several samples were cast onto a glass surface. The resulting graphene films, after removal of liquid, have a thickness that can be varied from approximately 10 to 100 μm.

The graphene films were then subjected to heat treatments that involve a temperature of 80-250° C. for 1-2 hours. This heat treatment generated sheets of graphene foam. Some of the graphene foam sheets were further deposited with a high-elasticity polymer, which acted to bond chemically bond graphene sheets together and also imparted additional elasticity (reversible compressibility) to the foam structure.

As an example, ethoxylated trimethylopropane triacrylate monomer (ETPTA, Mw=428, Sigma-Aldrich) was dissolved in a solvent mixture of ethylene carbonate (EC)/diethyl carbonate (DEC), at a weight-based composition ratio of the ETPTA/solvent of 3/97 (w/w). Subsequently, benzoyl peroxide (BPO, 1.0 wt. % relative to the ETPTA content) was added as a radical initiator to allow for thermal crosslinking reaction upon deposition on or in a graphene foam structure. The ETPTA monomer/initiator was then thermally cured at 60° C. for 30 min to obtain a protective coating layer on pore wall surfaces. An ionic liquid (PP₁₃TFSI) was then sprayed over this protective layer and heated to permeate into the polymer network.

The reversible compressibility of these graphene foam samples are typically in the range from 45% -85%.

Example 4: CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen, Z. et al. “Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapor deposition,” Nat. Mater. 10, 424-428 (2011). Nickel foam, a porous structure with an interconnected 3D scaffold of nickel was chosen as a template for the growth of graphene foam. Briefly, carbon was introduced into a nickel foam by decomposing CH₄ at 1,000° C. under ambient pressure, and graphene films were then deposited on the surface of the nickel foam. Due to the difference in the thermal expansion coefficients between nickel and graphene, ripples and wrinkles were formed on the graphene films. In order to recover (separate) graphene foam, Ni frame must be etched away. Before etching away the nickel skeleton by a hot HCl (or FeCl₃) solution, a thin layer of poly(methyl methacrylate) (PMMA) was deposited on the surface of the graphene films as a support to prevent the graphene network from collapsing during nickel etching. After the PMMA layer was carefully removed by hot acetone, a fragile graphene foam sample was obtained. The use of the PMMA support layer is critical to preparing a free-standing film of graphene foam; only a severely distorted and deformed graphene foam sample was obtained without the PMMA support layer. The graphene foams were then impregnated with a thin layer of lightly crosslinked polyaniline network. The reversible compressibility of these polymer-bonded graphene foam samples are typically in the range from 15% -55%.

Example 5: Graphene Foams from Hydrothermally Reduced Graphene Oxide

For comparison, a self-assembled graphene hydrogel (SGH) sample was prepared by a one-step hydrothermal method. In a typical procedure, the SGH can be easily prepared by heating 2 mg/mL of homogeneous graphene oxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at 180° C. for 12 h. The SGH containing about 2.6% (by weight) graphene sheets and 97.4% water has an electrical conductivity of approximately 5×10⁻³ S/cm. Upon drying and heat treating at 1,500° C., the resulting graphene foam exhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm, which is 2 times lower than those of the presently invented graphene foams produced by heat treating at the same temperature. The reversible compressibility of these graphene samples are typically in the range from 35% -75%.

Example 6: Preparation of Graphene Foams from Graphene Fluoride

Several processes have been used by us to produce GF, but only one process is herein described as an example. In a typical procedure, highly exfoliated graphite (HEG) was prepared from intercalated compound C₂F·xClF₃. HEG was further fluorinated by vapors of chlorine trifluoride to yield fluorinated highly exfoliated graphite (FHEG). Pre-cooled Teflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, the reactor was closed and cooled to liquid nitrogen temperature. Then, no more than 1 g of HEG was put in a container with holes for ClF₃ gas to access and situated inside the reactor. In 7-10 days a gray-beige product with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixed with 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected to an ultrasound treatment (280 W) for 30 min, leading to the formation of homogeneous yellowish dispersions. Five minutes of sonication was enough to obtain a relatively homogenous dispersion, but longer sonication times ensured better stability. Upon casting on a glass surface with the solvent removed, the dispersion became a brownish film formed on the glass surface. When GF films were heat-treated, fluorine was released as gases that helped to generate pores in the film. In some samples, a physical blowing agent (N₂ gas) was injected into the wet GF film while being cast. These samples exhibit much higher pore volumes or lower foam densities. Without using a blowing agent, the resulting graphene fluoride foams exhibit physical densities from 0.35 to 1.38 g/cm³. When a blowing agent was used (blowing agent/GF weight ratio from 0.5/1 to 0.05/1), a density from 0.02 to 0.35 g/cm³ was obtained. Typical fluorine contents are from 0.001% (HTT=2,500° C.) to 4.7% (HTT=350° C.), depending upon the final heat treatment temperature involved. The reversible compressibility of these graphene foam samples are typically in the range from 25% -65%.

Example 7: Preparation of Graphene Foams from Nitrogenated Graphene

Graphene oxide (GO), synthesized in Example 2, was finely ground with different proportions of urea and the pelletized mixture heated in a microwave reactor (900 W) for 30 s. The product was washed several times with deionized water and vacuum dried. In this method graphene oxide gets simultaneously reduced and doped with nitrogen. The products obtained with graphene : urea mass ratios of 1:0.5, 1:1 and 1:2 are designated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogen contents of these samples were 14.7, 18.2 and 17.5 wt % respectively as found by elemental analysis. These nitrogenated graphene sheets remain dispersible in water. The resulting suspensions were then cast, dried, and heat-treated initially at 200-350° C. as a first heat treatment temperature and subsequently treated at a second temperature of 1,500° C. The resulting nitrogenated graphene foams exhibit physical densities from 0.45 to 1.28 g/cm³. Typical nitrogen contents of the foams are from 0.01% (HTT=1,500° C.) to 5.3% (HTT=350° C.), depending upon the final heat treatment temperature involved.

Example 8: Freeze-Drying Process for Preparation of Graphene Foams

Elastic graphene foam structures were fabricated by using a freeze-casting process. The process began with preparation of graphene oxide dispersions using a modified Hummers method and further dispersed using a bath sonicator. In a typical fabrication procedure, 1.5 ml of graphene oxide dispersion was mixed with ascorbic acid (1:2 weight ratio) in a cylindrical glass tube. The mixture was then placed in a boiling water bath for 30 min to partially reduce the graphene oxide. The glass tube was subsequently placed in a dry ice bath for 30 min to freeze the mixture. After being thawed at the ambient condition, the mixture was then placed in a boiling water bath for additional 4 hr to further reduce the graphene oxide. The resultant graphene hydrogel was then subjected to dialysis and freeze drying. The obtained samples were annealed at different temperatures in air (≤200 ° C.) or in argon (>200 ° C.). The reversible compressibility of these graphene foam samples are typically in the range from 50% -77%.

Example 9: Evaluation of Various Lithium Metal and Sodium Metal Cells

In a conventional lithium or sodium cell, an electrode (e.g. cathode) is typically composed of 85% an electrode active material (e.g. MoS₂, V₂O₅, lithium transition metal oxides, etc.), 5% Super-P (acetylene black-based conductive additive), and 10% PTFE, which were mixed and coated on Al foil. The thickness of electrode is around 50-150 μm. A wide variety of cathode active materials were implemented to produce lithium metal batteries and sodium metal batteries.

For each sample, both coin-size and pouch cells were assembled in a glove box. The charge storage capacity was measured with galvanostatic experiments using an Arbin SCTS electrochemical testing instrument. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on an electrochemical workstation (CHI 660 System, USA).

For each sample, several current density (representing charge/discharge rates) were imposed to determine the electrochemical responses, allowing for calculations of energy density and power density values required of the construction of a Ragone plot (power density vs. energy density).

Three sets of anode-less lithium metal cells were studied: (a) first cell containing nitrogen-doped graphene foam; (b) the second cell containing graphene foam with no nitrogen doping; and (c) the third cell containing no graphene foam. The cathode active material used in all these cells was the well-known NCM-622 (lithium nickel cobalt manganese oxide). Charge-discharge properties of these cells indicate that the energy density and power density ranges of these cells are comparable. However, SEM examination of the cell samples, taken after 30 charge-discharge cycles, indicates that the samples containing a nitrogen-doped graphene foam layer have essentially all or most of the lithium ions returning from the cathode during charge being encased inside pores of the foam structure, having no tendency to form lithium dendrites.

In contrast, lithium metal tends to get re-plated on external surfaces of un-doped graphene foam in a less uniform manner. Further surprisingly, as shown in FIG. 4, the cell containing nitrogen-doped graphene foam as an anode protecting layer exhibits the most stable cycling behavior. The graphene foam without nitrogen doping also shows a relatively stable cycling behavior.

Shown in FIG. 5 are battery cell capacity decay curves of three sodium metal cells. One cell contains a pristine graphene foam and a sheet of Na foil as the anode active material, and NaFePO₄ as the cathode active material. An elastic polymer layer was disposed between the Na foil and the electrolyte. For comparison, a second sodium metal cell containing pristine graphene foam (but no elastic polymer protective layer) and a sheet of Na foil as the anode active material is also investigated. A third cell contains a Na foil, but no graphene foam and no elastic polymer. The cell having both a graphene foam and an elastic polymer layer shows the most stable cycling behavior. The battery cell containing no anode protecting layer suffers from a rapid capacity decay as charge/discharge proceed.

In conclusion, we have successfully developed a new, novel, unexpected, and patently distinct class of conducting graphene foam materials that can be used in a lithium metal battery or sodium metal battery for overcoming the dendrite and solid electrolyte-electrode interfacial gap issues. This class of new materials has now made it possible to use lithium metal and sodium metal batteries that have much higher energy densities as compared to the conventional lithium-ion cells or sodium-ion cells. 

We claim:
 1. An alkali metal battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between said cathode and said anode, wherein said anode comprises: a. An anode current collector, initially having no lithium, lithium alloy, sodium or sodium alloy as an anode active material supported by said anode current collector when the battery is made and prior to a charge or discharge operation; and b. a reversibly compressible graphene foam, comprising multiple pores and pore walls, wherein said graphene foam either substantially constitutes said anode current collector or is disposed between said anode current collector and said electrolyte and wherein said graphene foam, when tested under compression, has a recoverable elastic deformation or reversible compressibility greater than 5%.
 2. The alkali metal battery of claim 1, wherein said pore walls contain single-layer or few-layer graphene sheets, wherein said few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 2.0 nm as measured by X-ray diffraction and said single-layer or few-layer graphene sheets contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.001% to 25% by weight of non-carbon elements wherein said non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof.
 3. The alkali metal battery of claim 1, wherein said graphene foam is chemically bonded or physically attached to a surface of said anode current collector.
 4. The alkali metal battery of claim 1, further comprising a polymer that is disposed between said graphene foam and said electrolyte, impregnated into said pores, or coated on or bonded to surfaces of said pore walls, wherein said polymer comprises an elastomer, an elastic polymer, an electron-conducting polymer, an ion-conducting polymer, or a combination thereof.
 5. The alkali metal battery of claim 4, wherein said elastomer comprises a polymer selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, a copolymer thereof, a sulfonated version thereof, or a combination thereof
 6. The alkali metal battery of claim 4 wherein said elastic polymer contains a cross-linked network of polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.
 7. The alkali metal battery of claim 4, wherein said ion-conducting polymer is selected poly(ethylene oxide) (PEO), polypropylene oxide (PPO), poly(ethylene glycol) (PEG), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), cyanoethyl poly(vinyl alcohol) (PVACN), polyacrylic acid (PAA), a pentaerythritol tetraacrylate (PETEA)-based polymer, an aliphatic polycarbonate, poly(vinylene carbonate) (PVC), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), poly(trimethylene carbonate) (PTMC)), a single Li-ion conducting solid polymer electrolyte with a carboxylate anion, a sulfonylimide anion, or sulfonate anion, a crosslinked electrolyte of poly(ethylene glycol) diacrylate (PEGDA) or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.
 8. The alkali metal battery of claim 4, wherein said electron-conducting polymer comprises chains of a conjugated polymer selected from polyacetylene, polythiophene, poly(3-alkylthiophenes), polypyrrole, polyaniline, poly(isothianaphthene), poly(3,4-ethylenedioxythiophene), alkoxy-substituted poly(p-phenylene vinylene), poly(2,5-bis(cholestanoxy) phenylene vinylene), poly(p-phenylene vinylene), poly(2,5-dialkoxy) paraphenylene vinylene, poly[(1,4-phenylene-1,2-diphenylvinylene)], poly(3′,7′-dimethyloctyloxy phenylene vinylene), polyparaphenylene, polyparaphenylene, polyparaphenylene sulfide, polyheptadiyne, poly(3-hexylthiophene), poly(3-octylthiophene), poly(3-cyclohexylthiophene), poly(3-methyl-4-cyclohexylthiophene), poly(2,5-dialkoxy-1,4-phenyleneethynylene), poly(2-decyloxy-1,4-phenylene), poly(9,9-dioctylfluorene), polyquinoline, a derivative thereof, a copolymer thereof, a sulfonated version thereof, or a combination thereof.
 9. The alkali metal battery of claim 4, wherein said elastomer, elastic polymer, electron-conducting polymer or ion-conducting polymer further contains a lithium salt dispersed therein and said lithium salt is selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.
 10. The alkali metal battery of claim 1, wherein said graphene foam has a density from 0.005 to 1.7 g/cm³, a specific surface area from 50 to 3,200 m²/g, a thermal conductivity of at least 200 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 2,000 S/cm per unit of specific gravity.
 11. The alkali metal battery of claim 1, wherein said graphene foam has a density from 0.1 to 1.7 g/cm³, an average pore size from 2 nm to 50 μm, and a specific surface area from 300 m²/g to 2,600 m²/g.
 12. The alkali metal battery of claim 1, wherein said graphene-metal hybrid foam, when measured without said metal, has an oxygen content or non-carbon content less than 0.01% by weight and said pore walls contain stacked graphene planes having an inter-graphene spacing less than 0.34 nm, a thermal conductivity of at least 300 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 3,000 S/cm per unit of specific gravity.
 13. The alkali metal battery of claim 1, wherein the pore walls contain stacked graphene planes having an inter-graphene spacing less than 0.337 nm and a mosaic spread value less than 1.0.
 14. The alkali metal battery of claim 1, wherein said pore walls contain a 3D network of interconnected graphene planes.
 15. The alkali metal battery of claim 1, wherein said graphene foam is pre-loaded with lithium or sodium before the battery is made, or the anode further contains a lithium source or a sodium source.
 16. The alkali metal battery of claim 15, wherein said lithium source is selected from foil, particles, or filaments of lithium metal or lithium alloy having no less than 80% by weight of lithium element in said lithium alloy; or wherein said sodium source is selected from foil, particles, or filaments of sodium metal or sodium alloy having no less than 80% by weight of sodium element in said sodium alloy.
 17. The alkali metal battery of claim 1, wherein said graphene foam is under compression when the battery is manufactured.
 18. An alkali metal battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between said cathode and said anode, wherein said anode comprises: a. An anode current collector, an anode active material supported by said anode current collector, wherein the anode active material is selected from lithium, lithium alloy, sodium, sodium alloy, or a combination thereof; and b. a reversibly compressible graphene foam, comprising multiple pores and pore walls, wherein said graphene foam either substantially constitutes said anode current collector or is in electronic contact with said anode current collector and wherein said graphene foam, when tested under compression, has a recoverable elastic deformation or reversible compressibility greater than 5%; wherein said anode active material resides in pores of said graphene foam or in physical contact with said graphene foam.
 19. The alkali metal battery of claim 18, wherein said graphene foam is under compression when the battery is manufactured.
 20. The alkali metal battery of claim 18, further comprising a polymer that is disposed between said graphene foam and said electrolyte, impregnated into said pores, or coated on or bonded to surfaces of said pore walls, wherein said polymer comprises an elastomer, an elastic polymer, an electron-conducting polymer, an ion-conducting polymer, or a combination thereof. 